The main aim of Stop Killing Games is to ensure the practice of rug-pulling eventually comes to an end. They are not trying to save MMOs, for example.
Moreover they don't demand that every game currently on the market comply with open-sourcing requirements: at a minimum, companies always have the option of simply providing customers with adequate notice before shutdown. Open-sourcing the server would be nice, but it's hardly the only way to protect consumers' interests. Scott has, for example, suggested game boxes being marked with an estimated expiry date for online service functionality.
But most importantly: because this is about future games, not the present, the market has time to change. If studios and publishers are designing their games with a fair EOL in mind, then they can make decisions from the get-go to avoid licensing dependencies that they won't be able to release in a possible 'afterlife' version of the game. As suggested by your example of GameSpy in C&C: Generals, when a commercial dependency is crucial to a game's success, it tends to be a client-side library, but typically the problematic dependencies aren't crucial; they're e.g. add-ons for Unity or Unreal that the studio bought to save time. In a world with SKG laws, the providers of these dependencies aren't going to be a stagnant target either—demand for compliant libraries will motivate development of open-source versions.
Interestingly, the will for doing this does exist among game developers; they just need the institutional support from legislation to twist the arms of the studios and publishers. Ross Scott has talked to a lot of devs who are burnt out from having their projects cancelled, leaving them with huge gaping holes in their resumes and portfolios where they've spent years on unreleased projects that are stuck under NDA. In general they tend to see SKG as a path to ensuring the games that do see the light of day aren't also scrapped, which would erode their work histories even further. (Apparently it also just plain feels bad to have your work erased from history. Shocking, I know.)
