For better or worse, the biggest problem/asset of FFXI is its community. Tried to get into it a couple years ago thinking "Old School MMO! So I can really get social, talk with people in parties, level with folks, etc etc." So what was the game like? "Get Trusts. Level up. Get at least some of the good end-game gear. THEN you can join our parties." And I was like "If I want to play an MMO where I solo level and only do end-game content with people, I already have FFXIV for that..."
I actually like old school games. I tried whatever one of the more recent private servers for FFXI was for a couple of days and actually enjoyed THAT experience. Being a healer that mainly HEALS people, how avant-garde! I actually ENJOYED it...
...but I also realize it's a private server, and I largely didn't stick with it because of that. I kind of have never really trusted private servers, as you're at the whims of the people running it, the owning company not filing a cease and desist, and so on. I DID like how the install was so much better (seriously, how is PlayOnline still a thing in 2023??) and even controller support being natively active (OMG YES!), and wish the actual game/company would officially buy the project from those people and run it as an official thing.
I think that there are a lot of people hungry for an old school MMO like that. Large portions of Ashes of Creations' enthusiasts are, and Pantheon: Rise of the Fallen's entire community is. While there aren't 13+ million like at WoW's height, you could easily get 500k to 2M people for a project like that if someone does it well.
The problem with current FFXI is it isn't that. It's got that as a baseline, but with tons of changes over the years (some good, some bad) that have turned it into a Trust based leveling system. It's a solo-focused MMO (like FFXIV) built in an old school social MMO world and foundation...and that just doesn't work. It needs to pick a lane. If it picked the old school lane with a modern graphics engine and control scheme, I think that WOULD be pretty popular. But the current "old school look/new school mechanics and play" is just "play a modern MMO with a 20 year old graphics engine", which is just stupid. I'd play a 20 year old MMO with a 20 year old graphics engine, or a modern MMO with a modern graphics engine, or even a 20 year old MMO with a modern graphics engine, but I'm not going to play a modern MMO with modern seeming community but a 20 year old engine and mechanics. That's just stupid.