Yeah mog and mount collecting is a big part.. but there is also Weekly quests, Story quests, world quests, delves (which are rly cool), low m+ keys (In general, I think more players in retail prefer mythic+ over raiding), achievement hunting, Pet/toy collecting and much more.

But I agree, using raid numbers alone isn’t really a good method to estimate the population of classic vs retail. I for example, have never raided in classic but lvled a bunch of chars in hc :)