Yeah you're right! That approach is ephemeral though - you still have to then update your build config if you like it.
And it's just ... I don't know, it doesn't match my nixos mental model. But that again could be caused by me having it only on server environments. Perhaps I'll give nixos on a client a shot at my next job (currently locked in MacOS mainly which is such an annoying pain it's ...)
Oh and something that's different than from many other distros for me: upstream management is quite a bit slower, even on unstable:
a generation switch takes around 30 minutes for me - ironically hm switch even longer with flake updates.
Both are caused by software I follow upstream so close though that I have to compile it myself, that's not "normal" for most nixos users I assume :)