> It's not feasible to open-source all of that, especially given that it may well still be in use for more recent titles

If they're still running their authentication server (for example), then they wouldn't need to release that service.

Patching the game to no longer contact the authentication server would also be acceptable, for services that aren't a core part of the game. It's pretty likely the game already allows this for development/debugging.

If they've accepted money from people to buy the game, and don't want to keep the authentication service running, and don't want to patch the game to no longer require the authentication service, and don't want to refund people, and don't want to release the authentication service so others can run it - I think it's fair for a regulation to force one of those.