Another counterpoint to this is a world that exists despite almost no one playing it: Clanlord[1]. I started playing this game during its beta test when I was 16(I'm 32 now) where there were as many as 200 players on a single server. It predated every MMORPG you've ever heard of, unless you were into online games during their earliest days. I still log on there every now and then to "spend" my ranks (basically leveling up skills), and there are still afk clients consistently logged into the server which sit in the town center to heal folks. The coolest thing is the fact that its creator has posted patch notes[2] every 2 weeks for the last 16 years -- how incredible is that?
What's interesting about this game is how the creator of it solved a lot of very difficult technical challenges where there was no common solution at the time. There was always one computer running one persistent world, so he spent a lot of time working on scaling out the performance of that server to be able to handle a possible load of 200 concurrent connections, where the average game at the time could maybe handle 6-8.
Knowing what I know now about programming, I would love to be able to get under the hood of that game and play around with it.