That's not how it works. Steam still connects even when set offline and won't run if it or the game is out of date. And good luck permanently disabling your internet connection because Steam can't run like that forever.
I play games as old as 1994 natively on my 2060 Super using November 2019 build of Win10-64. DOSBox/emulated games can be even older dating back to the early 1980s.
This is a fantasy. We've already seen this where XP/Vista users are locked out of their games because Steam no longer supports those platforms. Soon that'll happen to Windows 7 users.
"Oh well you should just upgrade" is an invalid statement. If a currently working game has a minimum requirement of XP, why should should XP/Vista/7 users be forced to upgrade? If the mythical kill switch existed we'd have already seen it in action.
And how exactly do you think a bankrupt company will continue to supply cloud services for hosting tens of thousands of games for millions of user? Where will the funding come from?
Bottom line: for all practical purposes, if Steam goes down you've lost all of your games except for the rare ones that have no Steam DRM. Or the ones you can get working cracks for. Imagining any other scenario is delusion.
I started this thread to raise awareness:
https://forums.anandtech.com/threads/the-anti-drm-thread.2552393/unread