Broken according to who? I don't remember having seen a single thread dedicated to the old agro system and how suposedly broken it was in the last months or year...
In one week of the new system we have had several threads, all very active, of people complaining about how broken it is. Maybe the old one was not flawless but it did not create any kind of controversy and most people seemed to just be fine with it, mainly because it was only used in solo runs or by not so experienced players who had to pull mobs 1 by 1 or invis run past them not for fun or for efficiency but because they had to in order to success. You could choose to use it or not. Most peple decided it was not worth using in groups when you can just kill everything as fast as if you invised and much faster than pulling 1 by 1. Now it affects everyone playing experience solo and in group regardless if you want to take advantage of it or not. As some other posters suggested I suspect it is to adapt the game to reaper mode and this is a huge mistake IMO. Not all the playerbase wants to run reaper and they said it's not meant for everyone, and the player base is already short... Reducing playing options and styles won't help mantaining the ones still playing.
To sum up changing a not flawless but certainly not borken system when you have no one asking and everyone seems fine with it to a completly broken one that causes the forums to rage against it for a week doesn't look like a smart or productive move. Doesn't look like a fix either, more like the other way around