The WoW example is a little different from a subscription standpoint in that the server is a arguably major part of the game itself. The content you see, your character's data, world events, etc. all happen server-side. WoW is a lot more than just some netcode to get clients talking in a one-shot.

That being said, if Blizzard were to sunset WoW, then it should also be required to provide a way to self-host a server and a client update to connect to third party servers without needing to modify game files.

I'm not even saying they need to open-source it or make it free, just make a server application available.