If everything was on everything then sure, that would be great. But that’s not realistic, and having this alternative where everything is on Steam but not elsewhere is not a good solution for that because Steam will be shit one day, and already has quite a few shitty issues.
Honestly, people have this ridiculous view of PC gaming and Steam sometimes. Look around the world and show me where having one company having a massive hold over that sector has ever worked out well long term for consumers.