It's not the fact that people are doing it that's bad. It's the fact that the enrage mechanic itself is bad.

There's a reason it's called "enrage" after all. Many MMOs have them, and they're always a bad thing. Be it the boss now 1 shots everybody, mechanics get harder/faster, etc. Except this game, where failing to beat the boss in the required time actually makes the fight easier. And not just by a little bit, but by a lot by making the boss stop using every move it has except for a fairly weak aoe.