It’s still the best MMO entertainment on offer, although FFIVX is an alternative of sorts.
Your hypothesis that M+ is the sole culprit, doesn’t seem to hold up. When Blizz was independent and laser-focused on WoW (Vanilla to Wrath), they were at their peak in terms of creative design and capacity to build that design. What happened after Wrath? Oh wait, Activision bought out Blizzard and became ActivisionBlizzard in 2008. Now you have a business focused on profit and maximizing efficiency. The creative talent all took their golden parachutes and bailed out. I believe the guys running the show now really are doing their best, but they do not have the creative freedom that the original team from 2004-2008 had… they are under schedule/delivery deadlines and are asked to cut costs at every quarter, because it’s all about profit margin for ActiBlizz. The CEO could not care less about the games and the gamers, it’s only about what will earn more money for his stock options and for other shareholders.
That’s much more likely why we get “recycled old dungeons” as part of the current seasonal offering. M+ is very popular (15M participation as recorded in Raider IO Leaderboards and Blizzard’s own poll), so it’s not going away.
Thinking that M+ is the problem suggests you just don’t like it and anything you don’t like is a drain on dev resources that should go away. I don’t like PvP (since WoD), but I’m not naïve enough to suggest that one of the three pillars of end-content should just be deleted. 