I should note that Epic store is mostly viable because of Valve believing they can get things as locked down as Apple and Google and thus can charge the same 30

I should note that Epic store is mostly viable because of Valve believing they can get things as locked down as Apple and Google and thus can charge the same 30 % tax. "Alternate App Store for iOS" is never going to work, at least if someone doesn't break Apple monopoly. But that's not the case for PC games. So alternate store that charges less is obvious optimization.