This is the well known litany from string theorists to try and justify the inordinate amount of money threw at them to get back nothing of physical value: no falsifiable prediction.
Instead of reasoning on the worth of the effort spent in this direction to investigate nature (a very tangible companion) they try to steer the discourse toward this nonsense. We spent >50 years listening to these tales and the time has long passed since we are required to stop playing with these smoke and mirrors.
> justify the inordinate amount of money threw at them
They're theorists, you're paying for pencils and paper. String theory may not have produced a theory of quantum gravity yet, but neither has any other line of inquiry.
You are right, the amount of money spent in string theory proposals has been staggering if you take into account the size of the field. For decades competing (or even just non-aligned) research lines have been starved to feed this behemoth.
Particle accelerators have been built since way before any string theory was formulated.
The biggest and most powerful existing accelerator (LHC) has been built to fulfill the high energy/high luminosity requirements to explore the Highs boson energy regime (that has been found) and at most the lightest supersymmetric particles (not found as of today).
The Higgs boson is a cornerstone of the Standard Model. Supersymmetry is an extension to it that does not involve strings.
And right now we have no reason to think either would produce more useful math. Math at least is honest about studying math for the sake of math (or truth and beauty). Nothing wrong with throwing money at math, but physics is supposed to be about understanding the universe so if we only get math we get a bad return on investment no matter how nice the math is. Even if the math turns out to be useful elsewhere we didn't get what we wanted out of the investment.
They produced a family of theories with an infinite amount of underlying possible geometries, and we still don't know if these reduce in the low energy limit to the standard model. I humbly suggest this reading:
Instead of reasoning on the worth of the effort spent in this direction to investigate nature (a very tangible companion) they try to steer the discourse toward this nonsense. We spent >50 years listening to these tales and the time has long passed since we are required to stop playing with these smoke and mirrors.