That's the thing for most games --- the loyal old fan base is not playing. The virtual worlds are empty, the social hubs are ghost towns, matchmaking doesn't work because there is only one player online.
Even in games that are still very active like World of Warcraft, there are many remote zones, especially those zones for beginners and from early expansions, that are completely devoid of other humans. Once-bustling digital cities are vacant. Searches for other players come up empty.
Yeah, but 'keeping them happy' doesn't mean they ARE playing. People don't like choices being taken away from them or having things change. I have a few games where it really rather annoyed me when the multiplayer support was pulled. I hadn't played a game of Outpost 2 in at least a year or more when Sierra pulled the plug on its multiplayer support, but it would still have made me happier as a customer to know that 'something' was officially done for if I ever wanted to fire it up and have a game or two with some friends from way back when.
So having a "ghost town" server where you could log on once in a year to just have your fond old memories of a game bristling with live tainted by the empty tomb it has become thanks to almost no-one being online anymore? Would that really make you happy? Would you visit the places that meant something to you 10 years ago like you would visit the grave of a long dead family member, alone, just to remember?
Or would you fire up the game hoping it would be like you remembered it from 10 years ago, be disappointed, waste way too much time looking for players and that feeling of awe and wonders from 10 years ago, and quit dissatisfied in the end?
I think we all do from time to time crave to relive some of our old favorites... its easy with single player games, especially old console titles. Many of the games you fire up just to be disappointed by how bad the game actually aged... or maybe it was bad all along, you just remembered it differently. Some aged well and are still fun to play (hence my multiyear project of playing through my JRPG collection in my lunch breaks, and fill the gaps in the collection whenever a cartridge I don't own yet becomes available).
As frob points out, some game just don't lend itself to such nostalgic consumation though. After 10 years, it will be hard to get a population to an MMO that will make it able to relive its original intended state. MMO's live and die by the number of people online.
I wasn't really that impressed by WoW when it came out, and never tried it until some longtime friends finally got me to try in 2011. It was better than I thought and worse at the same time... cannot say I was dissapointed with, just could bring myself to play it enough to pay the sub, so I stopped playing after a month.
But the point is, I did start at zero where most of the population was doing there end game thingy. Apart from my friends starting new characters, I could have played a singleplayer game. The other players I met where scarce and busy getting into the end game action... and as a singleplayer game, WoW fails hard.
So while I do see that some multiplayer games could lead a long and happy life on private servers as long as there is a community dedicated enough to pay for it, and actually do the legwork, many MMOs are just not made to be played by a small community... without extensive changes, they will never be able to live up to their old glory this way.