The foldingideas videos about decentraland talks about this. "Dead" mmorpgs work on a small skeleton crew despite the original game having taken 100s of people years to make. Looking it up the turtle wow server was like 5-10k concurrent players? A lot for sure but bordering on that category.
It takes a lot of work to manage that but nothing compared to making an original IP from scratch.
Also, I'm not sure they have to start by creating an MMO
Instead of just building something you own.
this wasn't the goal of modding.
Honestly as someone building a game right now, I'd love to meet some people who are trying things and are open to meeting