FFXI isn't a great example here.
It is great FFXI is still running. However, the game is running on donated PS2 dev kits now. As soon as those PS2 dev kits bite the bullet (and they will), there are barely any working ones out there in the wild. The game is dead. It has come close to dying several times, but each time they have been able to source more dev kits. However, there are only so many times that can happen.
At some point, the engine will chug along but there is zero way to update it. As soon as it cannot be updated, it becomes a security risk and then POOF. Gone.