I was part of a team that invented "spacebase" back in the mid-1990s! We were able to support millions of simultaneous MMO players in the age when most people were using dialup modems and had vastly less bandwidth and vastly higher latency than they do now. This technology ended up being acquired by Sony, and used as part of the playstation network.
Like this company our work grew out of simulation programming done originally for the military (in this case the DoD) and like this company we provided an API and solution to rapidly partition the space so that the game client would only need to know about objects located near it according to in-game geometry. Like this product ours was fully distributed, etc.
Alas, we were ahead of the age of MMOs, though while World of Warcraft didn't yet exist, Ultima Online did, and there were a lot of other attempts at MMOs.
Nowadays people if there was less temporal difference people would say "They ripped us off!" but I can totally believe this company had the same idea... and they saw a green field because there were no competitors.
The problem is, there were no competitors because (at least back then) game developers were not interested in solutions they didn't invent themselves. Maybe that has changed.
I'd say that if you disregard indie development, that is still pretty much the case. There is certainly a bigger market for middleware, but I think it's generally met with skepticism.