Why are the strongest moves balanced by decreased/imperfect accuracy when they already have drastically power PP?

What if Gamefreak drastically lowered the PP of all moves but made them refresh at the end of battle so moves could be more balanced around potentially running out mid battle?

Barring that, what possible trade offs could higher power moves have aside from accuracy? What if say for instance Fire Blast did on average the same damage as Flamethrower, but it had a larger possible minmax variance?