I do think Steam can charge more because it offers more good services for users than any other app/game store: search/discovery features that have gotten better over time, per game communities/mods, tons of social features including a marketplace, multiple platform support, and their customer service. Sure their fee might discourage some developers, but if there is a case to be made for 30%, this is it. It all works because there is competition between Steam/GOG/Itch/Epic/etc.. on multiple open platforms in a pretty well differentiated way.