You're absolutely right, this is a larger issue (not just with games but with "always-on" and connected devices in general).

It's one thing for a company to shut down an older game's multiplayer-only services when the player base dwindles, but with many single player games now requiring a server connection to even function, what happens to those?

At the very least, companies should have a contingency plan for making a "sunset" patch available to allow those games to keep working - even in reduced capacity - once those servers go away.