I think I will have to disagree somewhat.
Difficultly should change how you play, not how long.
Needing to Grind better gear is not respecting the players time.
That said there are games I did enjoy the grind, "the world ends with you" is one of them, but that is a case of earning new ways to play in most cases.
Yeah thats kinda what i meant. Because of the increase in required DPS you will need to unlock harder to obtain gear with higher DPS which will bring your total playtime back to previous levels. But its a difficult to balance thing for sure.