generally when a game is created or any form of content there is a vision for it, the people who create it have some kind of expectation of how it will be digested, and this is a bit of an extreme example for dungeons i know but blizz designed dungeons to be done in a specific way and the AoE cap was on of the few ways they could have that idea realised,
it would be like going out of your way to watch a TV in a random order, im sure thats the not idea that the creators of a show would have and wouldnt be best pleased of it being all muddles and digested in the wrong way.
feel free to give any ideas on how this can be done, the only possible way is to make pulls impossible to do with more than a certain amount of mobs through random one shots and other such things that need interrupting, the only problem then is that there i just constant complaints about things regardless, basically everything these days is a lose lose situation for blizz, they can never do anything right because everyone has a different view/opinion on what is best.
and yet some of the capped classes like WW are currently the best AoE specs in the game, so i think it did its job reasonably well, the main problem with the AoE cap wasnt really balancing it was people found it less fun.