Since ProtonDB (and obviously Proton itself, Wine with its own WineApp DB, SteamOS) there is an easy way to check if your favorite games do work. That being said I understand that people are afraid. They might think "OK... well Elden Ring works but what about the DLC, or upcoming Elden Ring Nightreign?" and believe, probably rightfully so to be honest, that because Windows is still the most popular OS for gaming on PC and that game publishers are economically rational actors, more testing and fixes will be done against that target platform.
So... 100% is a ridiculous coverage because it's impractical but IMHO they are not that silly to "want" it. It's just a simpler way to say they are scared and do not want to bother. They would rather follow the crowd than take a risk themselves and be trail blazers.
All that being said now that ProtonDB exists and Valve is actively radically improving support via Proton, that gamers see in the wild SteamDecks popping up literally around them, in flights, airports, waiting rooms, etc they just can not ignore the fact that support is improving enough to have fun. Mentality will change but it takes time and Microsoft is fighting back because despite having Azure as their dollar printing asset, they are just hooked on bundling.