Truthfully I enjoy m+ and find it to be a really cool concept. I like having old content being made relevant yet there isn’t so much that it’s taking away from new things being created either. That’s a good balance. I don’t see why it is seen as a bad thing outside of the sellers and economy when lots of players participate in dungeons throughout the expansions lifespan anyways but now we can do it at challenging levels.
I will say in WoW, there is a huge difference in players doing something because there isn’t anything better to do in game and players doing content because they are enjoying it. WoW has some pretty devoted fans and it almost seems like many will continue to do things the most optimal way of doing them even if it’s not enjoyable.
The last couple of expansions for me personally have me doing things I dislike in order to acquire some of the items I felt were important to have. Doesn’t mean I like it but I did it lol.
I think it’s impossible to measure the contents success truthfully by those numbers. The numbers that are probably way more telling are how many players quit even before half way into the expansion and purchased the expansion.