Seeing as SE apparently agrees with this for other areas (sky, in particular), I see no real reason to keep this at 15 minutes. Personally, I don't even do sky, but ??? repop time has little to do with difficulty or skill.

In my opinion, it should be easy and fast to plow through old content, and if there's going to be timesinks in a game, they should be at the current, new endgame, not the endgame of 4 years ago.