I don't know if MMOs aren't truly slowing down, but FFXIV has certainly made an incredible turnaround, from the trash that was in 2010, to arguably become SE's best release in the last 20 years.
They aren't on the verge of being wiped out, but we only have a few popular ones today, unlike years ago, when we had plenty to go around.
Going by ESO's slated update for 2020, and their current expansion playing out with time, it seems they are trying to make a turnaround as well. WoW on the other hand is still in a muddy situation, and will likely have to use their next expansion as a means to turn things around on their end.
That being said, the only MMO I'm set to play for the rest of this year, will be WoW Classic, since I've been waiting for this for over a year now.