Stop Killing Games

As others have noted, the problem is often with keeping servers running. It's impossible to predict how successful a game will be in the long term, so the publisher can't make any claims to it's longevity. And, it doesn't make any sense to keep servers running if there's not enough income flow. Where this is really an issue is MMOs. Games like Forza already have some solutions: online features are eventually disabled, but you can still play the core game.

Open source the server infrastructure or describe the protocol in sufficient detail to easily be reverse engineered.

MMOs don't often have a profitability problem, they even tend to overstay their welcome compared to any other game. While it would be nice to get a server binary to self host after they're EoL it's gonna be unfeasible to run it anyway.

The issue is really more with lazy implementations where a server check is required to play something that's fully single player as you say, which has become standard for major publishers now and is far too common for indie games too. It's not too much to ask to do the bare minimum and keep that single instance auth server online or just remove the requirement entirely by commenting out a few lines.

This won’t require companies to keep servers running, just that they have an end of life plan, eg: releasing a version of the server that can be self hosted for multiplayer games

Don't keep the server running. Let others put up servers.

Either release the server binary or code or publish the bare minimum API spec so others can build a server from scratch. Strip away any proprietary stuff. And don't sue when other people have server up and playing your game.

That's quite some "just".

Dumping your server binaries onto a FTP server is a quite negligible "just"

Why would it be unfeasible? Plenty of MMOs have had private servers.

Mate, WoW Private servers were so profitable that Blizzard decided to ship their own. What makes it unfeasable to self-host?

Perhaps from a technical perspective, but rarely from a legal / IP perspective.