i think nothing beats literally getting the zip file with all the contents of the game with no middleware like GOG employs. to decentralize the store further requires the devs to at least manage their own website hosting, domains, ownership status accounts for updates. the only step available beyond that is the payment methods, and i don't think there's any viable solution to be done in that case besides having more companies like Stripe and Paypal.
in that sense, Itch is handling things pretty good for devs so far,