I'm curious what applications there might be if any in number theory. If I recall, langlands had motivations from string theory concepts which ultimately wasn't as successful as hoped in physics.
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Unless it's been firmed up a great deal in recent times.
Predicts that special relativity holds up at all scales (check, according to all evidence so far), predicts general relativity at low energy scales (check).
So it's false that it has no testable predictions. None of this happened "in recent times" though, it's been understood for a long time.
Imagine if someone had said 30 years ago that the "higgs boson theory" is a failure because we couldn't then perform the experiments to detect it.
I don't, thus neatly resolving your issue.
Neither does anybody else. Testable means something like testable today or in the designable future and it always has.
As for the fact this may mean a true theory is not testable today even though some hypothetical technology in the hypothetical future wielded by hypothetical beings could hypothetically resolve the problem, well, welcome to the universe we live in. This is not special pleading applied only to string theory. It's evenly applied to everything. It's just that string theory gets hit by this particularly hard, although not uniquely so (to the best of my knowledge, loop quantum gravity is also rather short on testable predictions). The only utility of quadruply hypothetical advances is to science fiction authors. And I've greatly enjoyed many such stories. But it's important to distinguish between science fiction and what we can do in reality.