The ecosystem experience does degrade. The Mac is not ideal, often requiring users to download apps from multiple sources, and use multiple goddamn launchers and update mechanisms. The problem isn’t that users can get apps from multiple sources, it’s that they are often required to, which sucks and bloats the system. This is what I fear might happen to the rest of Apple’s devices.
If everything would available from every single source, allowing consumers to choose one source, then everything would be great, but that’s sadly not the direction this seems to be going.