Some of the math developed for String Theory has found other uses, like developing better models of surface boundaries in solids. Very useful for the science of materials, it’s wrong to say its a waste. In it’s original use it very well could be a waste, but it’s a “tool” and creative people will find other uses for the “tool”.
I do not believe there are any materials applied-egineering processes that have found their answers to their problems using string theory. It is fine for explaining things to other string theorists, but of no empirical application.
The error of it stems from a logical error of some mathemeticians, when they think that if they can merely create something by the manipulation of mathematical formulas alone, then the universe DOES create it, somewhere/time. Again, the models start with factors the theorists imagines, because there is not an empirical evidence factor for it.
Its mathematical play, and very expensive, and very wasteful, but keeps some mathematicians employed, when they might turn their inquistive minds to more socially dangerous pursuits, cause they don’t have any other dam thing to do.