It depends what "stops working" means? I have an old Mac where Steam isn't officially supported anymore, but I can still run all the games that I installed on it. The Steam client simply stopped updating and the last version that worked still works fine. Put the Steam client in offline mode and it works.

Or you mean Valve shutting down Steam? Then yes, it could be a problem for some games that use Steam servers for online multiplayer. But, Steam is Valve's core business with proven track record and many loyal customers. It's much more likely that Stadia would shut down then Steam going away.

Not quite. With Steam you can download the game files. Yes getting them to run without steam is a little trickier, but its not quite the same as stadia. That's a total loss of any form of ownership.

Most of the time you can just launch the game executable and it works fine for most games

I cannot imagine casual player doing such things. It's only a theoretical possibility.

Any mass-extinction event for Steam would most likely mean an application/tool would be widely distributed which removes drm from their already-installed games.

You can download the files but it's not your game it's just some gamefiles that most likely won't work without steam / internet.

Tomorrow Steam shutdowns you lose every games, period.

No you don't. Devs don't have to use Steam's DRM if they don't want to, so you won't lose every game. Period.

Most games do use DRM. However, you can use Steam client in offline mode as long as you were last logged into your primary account. So, even if Steam shuts down, you can still run the games on your computer.

So, you really won't lose any game.

This is just blatantly false