You can find Steam cracks for most popular games on torrent sites right now.
So, realistically someone would make a tool to do this automatically (not done now because API changes quickly, which is not an issue at EOL).
The larger issue is multiplayer servers run by Valve. Some games allow you to run a server and connect to arbitrary servers, most do not. Those are going to be somewhat unplayable without explicit action from Valve.
I used to pirate games until Steam made them affordable and solved the lost CD/DVD issues. So I bought some games for my friends to make up for my past piracy of those games.
So, realistically someone would make a tool to do this automatically (not done now because API changes quickly, which is not an issue at EOL).
The larger issue is multiplayer servers run by Valve. Some games allow you to run a server and connect to arbitrary servers, most do not. Those are going to be somewhat unplayable without explicit action from Valve.