While you're not wrong, by that logic, it's actually fairly trivial to take my Steam downloads drive and run it on any computer even without my Steam account.
It works in the same way that dumping your GameCube games and running them on Dolphin works... It's quick and easy, but it's against the ToS and requires breaking DRM.
Steam's DRM is weak, and in some interviews some Valve developers even gave hints that this is on purpose. Many Steam games will simply run without Steam if you just double click the .exe in the install folder, and the vast majority that only rely on Steam's DRM can be opened by running a free "Steam Emulator" software that pretends to be an active Steam account with a correct license.
A lot of Steam games don't have any DRM, and most of the rest are pretty easy to strip.
Give it a shot sometime. Completely quit out of Steam, turn off your internet, and try running some of your older Steam games directly from the Steam folder.
I do this somewhat often when my kids are on my other computer playing games on my account and I still want to play something. It's a little trickier on Linux since you need something to run the Proton/WINE layer, so I mostly stick to Linux-native games in that pretty rare case.
It used to be that if anyone in the group was playing any game it would lock you out of playing anything else on the main account without kicking them off.
But they eased up on it now so you can both play at the same time as long as you aren't playing the same game at the same time.
So just make a burner account for you or for your kids and family share the library to it and now you don't even have to go offline unless everyone in the house wants to play BG3 simultaneously.
Really? I haven't tried that since they revamped the sharing thing. I have three accounts, one for me, my wife, and one my kids share, and they're all linked. Most of the time my kids use my account, but I can easily change that if it'll allow simultaneous play (on different games).
You can still play it but increasingly games are becoming very different from what you bought.
I've started noticing a disturbing trend. More and more games that are older being sold at steep discounts or "free to play" and simultaneously jampacked with invasive telemetry and/or ads/microtransactions. And since Steam won't let you play older versions, those games are effectively dead.
The most recent ones I've noticed are Riders Republic and Borderlands 2. Helldivers also introduced a bunch of new microtransactions years after it's launch.