It is not really failed. It is more like a mathematical tools being forced to be the physic law. Think of it like Newton law without knowing relativity. You probably keep modifying to match the relativistic result. Instead of finding Einstein GR, that modified Newtonian equations grew into incredibly complex to match reality. Then declared it is so because it looks so nice and symmetrical instead of having actual physical result supporting it. Currently ST is so complex nothing in it can be proven and yet supporters declare it can "explain" all other theories/result. And each time it cant, they just up the dimension to show the math now corrected to match the physical result. It is no longer science at this point...and it is pity so many bright minds wasted their lives enhancing this branch of physics (or at this point more correct to say mathematics).