I can’t particularly imagine why there is any appreciable Windows 7/8 user base outside of enterprise machines that are, like, running CNC mills and will just never need to be updated.
I can't say I'm interested in maintaining a win7 steam box at this point, but I do sympathize with holdouts, as I'm deeply disappointed (though unsurprised) with where windows has been going since then.
Especially if you play with friends that are on Windows, it’s the only way to guarantee you aren’t the odd one out not being able to try a new game. Or even play an old one after an update.
Steam doesn't use Electron, it uses the older CEF to integrate Chromium (but your basic point stands).