Right now, the user experience with Crossover is that you have to manage the whole thing of installing Windows Steam in a Wine bottle, then installing games within that second Steam installation, then dealing with the fact that Steam doesn't seem to like having two instances running on the same computer (my native Steam loses connectivity every time I start the Crossover instance).
Wanting Proton on Mac isn't about that specific fork of Wine, it's shorthand for wanting the user experience that Valve gives you on Linux.
As a comparison, before proton, you could run steam with wine under linux. Wine directx implementation was sufficient to make a quite a few games work just fine, but the experience was atrocious. You either had to install a new instance of steam per game or install everything under one bottle which didn't work well as you had to tweak the install per grame. Personally I used it just for one or two of games that I really wanted to play and could actually run outsisde of steam after installation.
Wanting Proton on Mac isn't about that specific fork of Wine, it's shorthand for wanting the user experience that Valve gives you on Linux.