I think Arkani-Hamed is right to identify the 1974 GUT hypothesis as the starting point that led the field into this wrong basin.As far as I can see, Nima has never made a discovery – or claimed a discovery – that would show that grand unification was wrong or the center of a "wrong basin". Instead, Nima made the correct general point that if you try to improve your state-of-the-art theoretical picture gradually and by small changes that look like improvements, you may find a local minimum (or optimum) but that may be different from the global minimum (or optimum) – from the correct theory. So sometimes one needs to make big steps.
Is grand unification correct? Are three non-gravitational forces that we know merged into one at a high energy scale? My answer is that we don't know at this moment – the picture has appealing properties, especially in combination with SUSY, but nothing is firmly established and pictures without it may be good enough, too – and I am rather confident that Nima agrees with answer, Peter W*it's classic lies notwithstanding. Even if we take the latest stringy constructions and insights for granted, there exist comparably attractive compactifications where the electroweak and strong forces are unified at a higher scale; and compactifications where they aren't. String theory always morally unifies all forces, including gravity, but this type of unification is more general and may often be non-unification according to the technical, specific, field-theoretical definition of unification.
Nevertheless, W*it made this untrue statement in his blog post and the discussion started among the crackpots who visit that website: Was grand unification the first "wrong turn"?
Funnily enough, the N*t Even Wr*ng crackpots get divided to two almost equally large camps. In fact, if this community ever managed to discuss at least this basic technical question – what was the first wrong turn in theoretical physics – their estimated thresholds would fill a nearly perfect continuum. For many of them, Einstein's relativity was already the collapse of physics. For others, it was quantum mechanics. Another group would pick quantum field theory. Another group would pick renormalization. One more clique would pick the confining QCD. Those would be the groups that deny the theories that are rather clearly experimentally established.
But nothing special would happen at that place. There would be "more moderate" groups that would identify the grand unification as the first wrong turn, or supersymmetric field theories as the first wrong turn, or bosonic string theory, or superstring theory, or non-perturbative string theory, or M-theory, or the flux vacua, or something else.
I've met members of every single one of these groups. Needless to say, as we go towards more far-reaching or newer ideas that haven't been experimentally established, we're genuinely increasingly uncertain whether they're right. But because we can't rule out these ideas, they unavoidably keep on reappearing in research and proposed new theories. It can't be otherwise!
In May, I pointed out that the criticisms of inflation are silly because the true breakthrough of inflation was to notice a mechanism that is really "generic" in the kind of theories we normally use and that have been successfully tested (presence of scalar fields; existence of points away from the minima of the potential; de-Sitter-like cosmic expansion at these places of the configuration space), and that seems to be damn useful to improve certain perceived defects of the Big Bang Theory. Although people aren't 100.000% sure about inflation and especially its technical details, they have eaten the forbidden apple and figured out that the taste is so good that they keep on returning to the tree and pick some fruits from it.
To a large extent, exactly the same comment may be made about grand unification, supersymmetry, string theory, and all these other ideas that the crackpots often like to attack as heresies. Even though we're not 100% certain that either of these ideas holds in the Universe around us, we are 100% sure that because these possible theories and new structures have already been theoretically discovered and they seem to make lots of sense as parts of our possible explanation of physical phenomena, a community of honest theoretical physicists simply cannot outlaw or erase these possibilities again. To ban them would mean to lie into our eyes.
That's exactly what the N*t Even Wrong crackpots want to do – they would love to ban much of theoretical physics, although they haven't agreed whether the ban would apply to all physics after 1900, 1905, 1915, 1925, 1945, 1973, 1977, 1978, 1984, 1995, 2000, 2003, or another number. ;-) But they're obsessed with bans on ideas just like the Catholic Inquisition was obsessed with bans on ideas. This approach is fundamentally incompatible with the scientific approach to our knowledge.
New evidence – or a groundbreaking new theory or experiment(s) – may emerge that will make some or all ideas studied e.g. since the 1970s irrelevant for physics of the world around us. But because such an event hasn't taken place yet, physicists simply can't behave as if it has already taken place. In particular, no new physics beyond the Standard Model has been discovered yet which makes it clear that all conceivable theories of physics beyond the Standard Model would suffer from the same drawback, namely their not having been proven yet.
By the way, the disagreement about the identification of the "first wrong turn" is completely analogous to the "continuum of creationist and intelligent design theories" as it was discussed by Eugenie Scott, an anti-creationist activist.
Just like you can ask what was the first wrong turn in high energy physics, you may ask what is the first or most modest claim by Darwin's theory that is wrong – or the most recent event in the Darwinian picture of the history of species that couldn't happen according to the Darwinian story.
If you collect the answers from the critics of evolution, you will find out that they're equally split as Peter W*it's fellow crackpots. In fact, the hypothesized "first wrong statement" of the standard picture of the history of Earth and life may be anything and all the choices of these wrong statements fill a continuum – they cover all statements of cosmology, geology, biology, macroevolution, and microevolution that have ever been made.
Some people deny that the Universe is more than thousands of years old. Others do accept it but they don't accept that life on Earth is old. Some people accept that but they claim that many "kinds" of animals and plants had to be born simultaneously and independently because they're too different.
In general, "kinds" are supposed to be more general, larger, and more Biblical taxonomic groups than "species" – although "kinds" isn't one of the groups that are used by the conventional scientific taxonomy. However, when you ask how large these "kinds" groups are (questions like whether horses belong to the same "kind" as zebras), various critics of evolution will give you all conceivable answers. Some of them will say that "kinds" are just somewhat bigger than scientific species (those critics of evolution are the most radical ones and many of their statements may really be falsified "almost in the lab"), others will say that they are substantially bigger. Another group will say that "kinds" are vastly larger and they will "only" ban the evolution that would relate birds and lizards or dinosaurs and mammals etc. These "most moderate intelligent designers" might tell you the same thing as the evolutionists concerning the evolution of all vertebrates, for example, but they still leave some of the "largest division of organisms" to an intelligent creator.
The actual reason for the absence of an agreed upon boundary is obviously the absence of any evidence for any such boundary. In fact, it looks almost certain that no such boundary actually exists – and all life on Earth indeed has the common origin.
Summary: continuum of alternative theories shows that none of them is defensible
Again, to summarize, critics of theoretical physics just like critics of evolution form a continuum.
All of them have to believe in some very important new "boundaries" but any specific location of such a boundary looks absolutely silly and unjustified. Some critics of the evolutionary biology say that zebras and horses may have a common ancestor but zebras and llamas can't. Does it make any sense? Why would you believe that two completely analogous differences – zebra-horse and zebra-llama differences – must have totally, qualitatively, metaphysically different explanations? Such a theory looks extremely awkward and inefficient. Once Nature has mechanisms to create zebras and horses from a common ancestor, why shouldn't the same mechanism be enough to explain the rise of llamas and zebras from common ancestors, too?
The case of the critics of physics is completely analogous. If grand unification were the first wrong turn, how do you justify that the group \(SU(3)\times SU(2)\times U(1)\) is "allowed" to be studied in physics, while \(SO(10)\) is already blasphemous or "unscientific" (their word for "blasphemous")? It doesn't make the slightest sense. They're two groups and both of them admit models that are consistent with everything we know. \(SO(10)\) is really simpler and prettier – while its models arguably have to use an uglier (and more extended) spectrum of matter (the new Higgs bosons etc.).
Well, the only rational conclusion is that the efforts to postulate any "red lines" of this kind are utterly stupid. Biologists must be allowed to study arbitrarily deep evolutionary processes and theoretical high energy physicists must be allowed to study all ideas that have ever emerged, look tantalizing, and haven't been ruled out. And critics of theoretical physics must be acknowledged to be intellectually inconsequential deluded animals.