100% this. When it was announced, it got massive backlash from the sweaties because “panda”. It just happened to be announced around the time a certain panda-based film was released, so without looking further into it people just assumed it was a rip off from that. I mean it wasn’t a perfect expansion and it did suffer from some early set backs like the 1000 dailies and reps locked behind reps, but those who actually played it seem to have enjoyed it. We also shouldn’t forget about the transition from 10/25m raiding to 20m which forced a number of guilds to merge (which always goes well).
ICC lasted a year too, and WotLK had the famous “it cost us a raid tier” gap in addition to Naxx recycling. DS lasted a long time too. Content droughts were pretty normal at that time. Admittedly they don’t help, but don’t think this was exclusive.
5 years is still pretty good, and there are likely a lot more factors involved in that. A large volume of players who were maybe 12-18 when Vanilla came out were now in that early adult stage of life where things change. People go off to university, begin relationships, discover parties, get jobs, have children. These take considerable time away from gaming, and I’d argue that a number of players in that age bracket decided that a long-term investment in a video game wasn’t worth it at the time. MMO’s a whole at that time were in decline and new things like MOBA were making a rise.
MoP wasn’t perfect, and it did have its problems but it was generally a very solid expansion. Zones were beautiful, the story was interesting, raids were fun, scenarios were a new feature (I still don’t get why they were removed), classes were fun. I think the people who look back now and didn’t enjoy MoP either quit after a couple of months or only remember spending 12 months in SoO.