I don't see the long term success you talk about anywhere. It lasted 2 years like any other expansion, folowed by two very underpar ones.
Also the changes to endgame promoted a "must get everything immediately then leave" mentality, mythic ilvl gear is easily obtained by everyone, and content that was good removed (like mage tower). Now every expansion from Legion onwards has a sort of timegated farming system where you progressively unlock your character power which is really sad, instead of actually going into endgame activities or open world. More "parasitic" systems as you said that live in a bubble and we do because there's something we need in there and not because we want.
Legion worked well at the time because there was WoD before. It introduced design choices that are clearly making the game worse for a lot of players, given its current state.
We'll see what 10.0 will bring. I was more than happy with the classic "do your stuff once and then there's the raid". M+ is only to keep aswell cause it's really popular but it needs to be its own game mode and not a way for people to get powerful gear for raiding (and also the opposite needs to happen).