For me, it'd also need a Linux compatibility layer on par with (or exceeding that of) Steam. On paper, I'm not a fan of Valve's exclusive hold on that market, but in practice nothing has come close for me so far (that I know of, at least).

I tried Lutris and Wine, but I had difficulties getting stuff to run, and the fixes required patience and some level of technical understanding (of Wine, specifically, not just Linux in general). They just don't have the same (comparatively simple) convenience of "check ProtonDB before you buy it, download game, run it, and usually it'll work fine".

The more advanced fixes usually involve nothing more than a few well-documented steps like copy/pasting a launch command, selecting something in a dropdown or downloading and extracting a file into some directory. It's not a universal "It Just Works", but I feel like it's been getting better and better, and that's just a headstart any competitor would have to work really hard to catch up with.