Can't do that if it's not your server. Don't we all love "the cloud" :)

I think open sourcing the clients is easier because it's always possible, it takes the dev almost no time at all (compared to newly creating infrastructure documentation), and players don't have to set up their own infrastructure (which would probably require a lot of time as well as skill) to play offline. The downside is that server functionality would need to be recreated.

If the game is open, you can just patch out the server dependency. Is the update, "cloud" saving, or online-friends functionality crashing the whole game on startup? Stuff not actually needed to play in single player? Comment out the line where it calls that function, maybe mock a few variables, and you're all set.

A friend of mine makes a game with offline play being possible, but the main value is in the community: custom levels and online play. It's all cloud magic with google cloud this and google cloud that. Good luck pulling up that infrastructure in 20 years (having to set up a mini google datacenter, even if the components are open sourced by google in the first place, which they're almost certainly not when "sun set"). The game tries to reach the server on startup and should detect when you're offline (for me that doesn't work reliably, but firewalling google play services has weird effects on many apps), but if you just remove those calls from the game altogether, the offline parts will work with no dependencies and you can just exchange level files instead of having an online browser. Everything but realtime multiplayer would still be possible. Customizing the client code to work with much simpler infrastructure is also likely faster than trying to replicate the "cloud" setup.