I wonder how they'll handle user reviews. Maybe performance won't be an issue, I know some games run even faster than on Windows, but bugs that you don't know are Proton specific could lead to bad reviews.
Excited as I am about a big player pushing Linux, we mustn't forget that the intent is almost certainly just to chip away at Windows and the threat it poses to Valve's own monopoly (as a PC game distributor).
Valve's behavior suggests that they've never really sought a monopolistic position. Vive was open to everyone, there's no exclusivity agreements with their marketplace, etc. But Steam requires a platform to sell games for, so making sure there's some kind of alternative is in their best interest.
In that sense, the more bad reviews the better!