M+ isn’t popular it’s a chore that most people don’t want to do but they want gear. So that is why M+ is the most toxic environment in the game. Just like other chores in game that people don’t want to do but Blizz gatekeeps gear behind it. Don’t mistake popular with forced chores. You gonna tell me Korthia is popular next 
That is a huge reason why M+ is bad for the game. It should be for competition for people that like M+ and also it would be better competition. Just like Arena shouldn’t reward better gear, it should be a place to compete.
Don’t matter this game is dead for a lot of reasons. Gearing in SL is the worst and M+ will only keep driving players away. If you like it cool you can like it with a small population while the majority of WoW players left for other games that don’t have chore list they are forced to do before the fun starts.
They should get rid of M+ and make a good game again that has actual content.
Most players don’t want to waste their time doing repeat dungeons, that isn’t content or fun.