Your assumption is that the given person/group CAN kill stuff. The WoW team isn't worried about people doing this on every floor. They're worried that when players hit the limits of their skill/gear, instead of saying "Let's end this run, go and get more gear/try different tactics", players will instead just wait for all their cooldowns for each pack, and slowly grind their way higher up.
Players are well known for doing every silly, stupid strategy they can to try and get ahead once they've hit their limits.