This argument would be a great one in 2016.

Now though, proton/wine works more or less for everything, and the storefront is a web based one anyway.

I'd hope this community of all places would understand that "just integrate X with Y" is never as simple as "just". It's still something a team needs to do, and the gain is minimal unless Epic is also going to try and make their own console-esque device. That's the incentive for Steam.