After that no further internet connection is required for either.
Really? Try copying that game out of your Steamapps folder and running it without Steam. Any game that uses Steamworks, which is the vast majority of them, will fail.
Try keeping an archive of that game on a USB stick or something. Try installing it without the Internet. If it uses Steamworks, well, good luck. You'll fail.
You are trying to equate downloading a game with requiring the Internet in order to "activate" or play a game. The two are not, and never will be equivalent, regardless of how much you believe it to be so.
Congrats, you've found an opinion piece with no hard data and a Google search which brings up a bunch of articles stating that removing DRM did not reduce piracy rates.
If Valve wind down, they'll issue the patch. If Valve or their creditors sell Steam then the service will still be operational.
Your blind faith is admirable, but naïve.
I will never give money to Valve, and I will enjoy their death if it happens. Steamworks, Origin and all the others are what is wrong with the AAA videogames industry. It deserves to die.
Really, what stake do you have in continued DRM-encrustation of toys? Why are you so in favour of it?