"eat shit. billions of flies can't be wrong."
sub count really doesnt mean anything, but lets assume for a second the only scenario where it can mean something.
where did all these players go to? FFXIV? guilds wars 2? black desert? blade n soul?
players didnt quit WoW to play a better MMO, they quit WoW to play something else entirely. as much as burger king is great, if you eat it every week for years you're probably gonna get sick of hamburgers regardless of who makes it.
MMO had its peak generation a few years ago and now that everyone of that generation has tried what it has to offer they jumped boat to a different genre (MOBAs). of course there are other factors involved here such as LoL and Dota 2 being completely f2p while WoW not only costs money but is substantially expensive at that for a game, but since there are quite a few f2p MMOs out there too that dont see any popularity compared to the top MOBAs i believe money is not the primary factor.
MOBAs might follow the same course as MMOs in the next 3 to 6 years, but wether the players will migrate back to an already existing genre or only if its a completely new one remains to be seen.