Instead of "if sold on the understanding that they will remain playable indefinitely" should be switched to say unless they are sold with an understanding that they will not be playable indefinitely.
Game companies should be explicitly stating whether a game will have a limited lifespan based on things like server availability. Especially for single player games with online verification.
If the use the word buy or own. "Buy now" "buy here" "buy XXX" That is purchased indefinitely.
If they are being rented for a limited time I needs to be explicitly stated as "rent" any mention of buy or discrepancy has the above mentioned purchased indefinitely.
I'd much prefer companies to be forced to release the source code for multiplayer servers once they decide to shut them down. There will always be fans who'd keep it running.
I would prefer any game that is no longer sold to fall into the public domain, including releasing the source code. Reward them for their limited copyright and pnly keep those protections as long as they maintain the game'a availability.
Nobody would buy a game that says it is only guaranteed playable for one day.
What they need to clearly state are expectations on planned lifetime of authentication servers, any specific technology that is required, and so on. Like people know multiplayer requires servers, but something that says they will have those servers for X number of years would help set expectations and encourage companies to plan long term support for games that might not be massive hits.
For single player games this would discourage terrible DRM that keeps games from being played just because authentication was retired.