If you actually look at the MMO/MMO-adjacent landscape... most of the ones that are still alive are over a decade old with some legally allowed to drink.
- Runescape: 2001
- WoW: 2004
- LOTRO: 2007
- Star Wars TOR: 2011
- Guild Wars 2: 2012
- Final Fantasy MMO (that people actually liked): 2013
- Old School Runescape: 2013
- Elder Scrolls Online (about that...): 2014
And a lot of that has to do with people increasingly using MMOs as "comfort games" which... work best for the oldies. And most of the money going into game dev focused more on annual games and then live service games where you still spend about the same amount of money per year but feel better because it isn't a monthly subscription. Instead of spending 10-15 bucks a month you spend 30-50 bucks every other month for the battle pass or the new expansion or the Officer D.Va skin and so forth. TOTALLY different.
So most of the newer MMO-ass MMOs are either doomed from the start for being overly niche or take a more Korean or Gacha oriented balance and monetization scheme where... they are competing against the juggernauts that are also closer to a decade old than not.