Very true. At most we can say that these guys found a mathematical theory that is consistent with what the universe looks like. I imagine that there could be an infinite number of such theories, if you are creative (and good at math) enough.
What if you considered (mathematically) the set of ALL theories that satisfied certain basic criteria (requires a small number of assumptions compared to the number of outputs it produces, doesn't change "arbitrarily" at some point in time... things like that) and which also matched the currently accepted physics of general relativity (in the limit of large sizes) and the currently accepted physics of quantum mechanics (in the limit of small sizes). And what if, when considering that set of theories, you were able to prove that any theory matching those criteria had to be equivalent (isomorphic) to a certain theory.
(This is like saying that the theory of basic algebra using equations and the theory of curves on graphs using geometry can be proved equivalent (isomorphic) by defining how to graph an equation and how to derive the equation for a graph. Once you've done that, it hardly matters whether you think the algebra is the "real theory" or the graphs... they are both equivalent and thus both equally "real".)
Suppose you found that, and thus proved (mathematically) that string theory was isomorphic (in certain key ways) to any theory that could apply to our universe, assuming that quantum mechanics and general relativity are roughly accurate. Why, then you WOULD actually have demonstrated that string theory isn't a "waste of time", that it is a useful way to make predictions about our universe.
And if I understand the article, this proof is precisely what these researchers are attempting to do. So far, they have constructed such proofs for much simpler universes than ours and a way of slowly working their way up to proving it for our universe.
That seems unlikely, because for that to hold they also need to show that there is nothing better than relativity and QM to describe the known universe. Then, you are back to square one, finding a better unifying theory, or even worse, proving that no such theory exists.
I don't think that you've really understood the sort of limits that the previous commenter (and the original article) were trying to describe. To a very good approximation, QM does describe our universe: any "better" theory must necessarily be equivalent in the appropriate limits (to many decimal places of precision, I might add). In the same way, relativity does describe our universe, and any "better" theory must be equivalent in the (different) appropriate limits. As I understand it, that is the level at which those are being assumed in this argument. The whole point of the theorems that these groups are trying to prove is that any "better" theory that matches both of these limits must be equivalent to string theory.
I think that would be even harder to achieve than a simple unification of QM and Relativity. Who knows what kind of phenomena we don't know about? There could be millions of theories that are consistent with current physics and still possible, given particular observations. In a few words, nobody can anticipate the kind of phenomena that could be observed by future physicists.
Except the history of such proofs suggests we not take them too seriously, because they necessarily depend on assumptions that may or may not apply to our universe. Hawking's proof that our universe contains a singularity in its past depended on a positive pressure assumption that is violated in inflationary universes but that was believed to be compellingly reasonable at the time, for example.
The important thing is that our inability to imagine a universe that violates some assumption tells us nothing about the likelihood that the universe violates it. What we can or cannot imagine is simply irrelevant to the way the universe actually is. No one could imagine a universe in which anything like Bell's Inequalities could be used to test local causality until Bell derived them.
There was nearly 200 years of philosophy arguing that no such test could possibly exist, culminating with the positivists just decades before it was found, and 2000 years of philosophy before that arguing that no such universe as one in which Bell's Inequalities were violated could exist, because no one could imagine such a universe, or such a test.
This is why science and math are different disciplines. Science is the discipline of publicly testing ideas by systematic observation, controlled experiment and Bayesian inference (if you read that definition carefully you'll find it's a lot closer to "Anything goes" than Popperian hypothetico deduction). Math is the discipline of formal deduction from axiomatic premises, or something like that. Math is an incredibly useful tool in science, but so is plumbing.
The attempt to prove there is one consistent theory of the universe is ambitious and interesting, but there are good reasons many scientists are skeptical about it. It has resulted in a great deal of interesting math and no strongly testable predictions over the past century (string theory encompasses so many diverse models of the low energy world that its predictions only ever knock off models, not the family of theories proper).
Testable predictions come in many forms, and "unique consistency" is one of them. It's just not one that many people outside the string community think they can achieve, and my personal bet is that the next century of string theory will involve a widely-believed claim that the uniquely consistent theory has been found, followed by its collapse some time later as a key assumption is found not to be true of the universe it is supposed to describe (that is if loop quantum gravity doesn't take the field on an experimental basis first.)
In principle, nothing that is unproven is impossible. But your belief in this possibility in particular shows that you are the one with a huge imagination.
