However, as far as anybody can tell, there is a definite downwards trend and the biggest player, World of Warcraft, has been hemorrhaging subscribers for years, currently down to an estimated 5.5 million from a peak of 12 million in 2010.¹)
It used to be that the MMO playerbase would migrate. A new MMO game would come out and everybody flocked to the new hotness, and if the game was any good, they'd stick. But World of Warcraft changed that. People still bought new MMOs as they came out, but they played them precisely as long as the number of months of free subscription included in the box, and then moved right back to WoW, leaving the challenger to wilt and eventually switch to a free-to-play model.
However, the droves that are leaving MMOs the last couple of year don't seem to be going to other MMOs, they just evaporate.
The people that enjoy raid-style content with a large multiplayer component probably end up in MOBAs²) like LoL (League of Legends) and DOTA (Defense of the Ancients), which offer all of the tension and drama of typical MMO end-game content without any of the boring grinding needed to unlock it. And according to Steam, DOTA 2 is the most played PC game in the world.³)
Others have moved to MMO-FPS games like The Division on PC and Destiny on consoles, and the rise of casual and mobile gaming coincides with the fall of MMORPGs. But as far as I can tell that still leaves millions of players that simply stopped playing.
My guess is that the world is just sick and tired of MMORPGs and I think it will take the death of WoW and a few years without any big MMO releases before a new generation of players can make any new MMO popular.
¹) https://www.statista.com/statistics/276601/number-of-world-o...
²) Multiplayer Online Battle Arena.