Correct me if I'm wrong, but if you go offline for any reason you're still able to play games in your library in Steam, isn't that so?
It could be. I have not tried this feature during the past 4 years. If I recall right, it works if you anticipate that you have to go offline, and restart in offline mode while you are actually online. Otherwise, Steam might not start.
Moreover, it depends on the games: one of my most played games (
Gremlins Inc.) has an always-online DRM, many games are focused on non-local multiplayer aspects which require an online connection, there are the GaaS like Dota2, or if understand right, Destiny 2.
There are games which might be worth buying on Steam (indie games, games with little to no DRM, etc.) for the reason you mentioned, and there are games for which Stadia may add value to your experience (games which require an Internet connection for any reason, games which your PC cannot run at 60 fps, games which have a restrictive DRM anyway, etc.).