You’re backups wont work if valve ceases to exist, since you need to be logged into a steam account, that owns the games, to restore the backups.
Same reason my D2D games ceased to work, cause D2D went away, along with their authentication servers.
That hasn’t been true for like ten years now.
Obviously anything that needs an internet connection will require steam. But pretty much almost all single player games do not need steam to run.
No you dont? You can literally just copy files onto an external hard drive, there is no requirement to use steam to do that.