1st, ppl here are talking about speed run with all classes
2nd , even if it is possible to clear all speed runs with all classes, I think everyone will agree that blm own dungeons and mnks own Mistress. So why would anyone bother to do harder clears of speed run (just for "challenge") and risk every run the speed run chest with that crappy drop rate? I can agree that doing a speed run chest only with all classes is perfectly doable, but again, why would ppl bother to do that and get only 2 chests instead of 5? Srsly , I think even if a pt can complete speed run w/o stacking classes, I don't think they will chain with that setup to get the drops.

and 3rd (has nothing to do with class stacking) I think SE makes speed runs only because they a too lazy to make a though dungeon that last 1h so they just make a dungeon that takes a maximum of 35mn (35 mn without rush) and they add 25 mn to the timer. imo, speed runs are just SE lazyness