Of course, so the initial product was more grindy than what it became. That's called improved upon. So you can't broad stroak it with how you perceived it at the beginning.
Mythic+ was and is engaging, so yeah, spamming the same dungeons over and over is appreciated by much of the playerbase actually. Too much for my liking, since it's now affecting other parts of the game like PvP in a negative way. I'm still not arguing against its popularity.
Alright, the mythic+ system and TBC heroic system aren't really comparable. The closest we had to M+ before were the challenge mode. WoW suffered from having the most difficult activity drop the best gear and only be of value to that same activity. Mythic+ made it so dungeons are always relevant, the drops are always relevant and obtaining the best gear is relevant outside of the same activity. Heroics eventually became obsolete, like you mention WQ's became in Legion. But you also refer to the endless AP grind, so by that logic they never became obsolete. You illogical people are weird, man.