It is not. Most of the full online game stacks are not open source and most of the open source ones are poor and under featured

That could change once a law comes out requiring all new developments to be designed differently. Besides, no one is talking about open sourcing the server code. Releasing binaries and patching the client to talk to a local instance is perfectly acceptable. A developer would then just need the ability to redistribute compiled builds.

That was my initial thought as well, but on second thought it does not seem like such a good idea to provide binaries that will never get patched for a public-facing service. Sure, not the company’s issue anymore, but still…

It might make a community rewrite of the server code easier, but that would likely only be attempted for very few games.

It doesn't need to be public-facing. The point is not to be able to continue playing as if the game was still being supported, it's to be able to continue playing at all, in some way. You can set up a box at home and let two or three of your friends in through a VPN. Security is not a concern.

That’s a fair point

A lot of things are costly until they become required by law - then vendors have no choice but making that part of the price they compete on.