The server costs are likely trivial compared to the employee cost. They'd need to retain support staff, both for the technical side (server maintenance) and in-game moderation and customer management.
I've worked on a number of medium-to-large systems. They're literally always held together with duct tape and prayers. The entire thing would probably collapse in a few days if someone weren't there babysitting the storage infrastructure alone.
I suppose it depends on how robust your infrastructure is. I know one place I worked, we sunset a game and essentially there was only one person supporting it part-time; with events being automated through a calendar. I'm sure if Apple or Google were to make some sort of major breaking change that would require engineering time to keep the game running they'd shut it down, but otherwise AFAIK it's still running on autopilot about 8 months after they made that decision.
I've worked on a number of medium-to-large systems. They're literally always held together with duct tape and prayers. The entire thing would probably collapse in a few days if someone weren't there babysitting the storage infrastructure alone.