I'm certainly a lay person here, so take this with a grain of salt. But my understanding is that this is part of the problem, or more the issue that people criticize.

I think it's largely uncontroversial that the math in string theory could be useful in other areas. But if that's your argument for the legitimacy of string theory then the question arises what string theory is and if it is still part of physics. Because physics has, of course, the goal of describing the real world, and, my understanding is, string theory failed to do that, despite what many people have hoped.

If string theory is "just a way of developing math that can be useful in totally unrelated areas", it's, well, part of mathematics. But I don't think that's how the field sees itself.