I'm not sure valve deceived you. It's not fair that we can't run purchased old games on the OS they were built for. they could really show instructions on how to make them run on that OS, maybe even make a simple but official lightweight client that can download it for you, on that old OS.

but if you are on windows 10, what can they do with a game they sold you that won't work correctly on anything beyond XP?
yes, the above things they could, and should. but even today you are not locked out: copy the game files to USB, drop in the goldberg emu, and play the game on your XP machine. It's a single file, not eben needs internet.
if the game had DRM? I am not sure that's the fault of valve. didn't the devs put it there?

and if you accept the "solution" to drop steam, and start renting your games? you won't be able to do even this (edit: because they have real drm, not measly steamdrm that's easily stripped out). you are literally locked out both if you stop paying, and if the service stops making that game available because their license expired, politics, or whatever. and you literally can do nothing about that.

but wait a minute. when, and how did exactly valve popularize always online DRM?

you know that they have nothing to do with denuvo, and steamdrm is not always online, right?