the issue is that the items you get isnt better then stuff you can easily obtain for 1/20 of the time invested and you need that gear to even do the last stuff. so all your doing is running a instance with insanely hard bosses 200 times just for a skin, not many people will be willing to do this which means very few people will even do the instances making it hard to even find a group and impossible to find good groups which can actually do the whole arah run for example.
i think that the items you get from doing the hardest stuff in the game should have a little better stats then the items you get from doing something very easy or just buying them from the market.
it would also put new life into the dungeon running and make people actually find new tactics to kill the harder bosses, instead of forcing a nerf to them.