This is the part I wish could be solved. I hate having to input billing into various websites and apps and not being able to track licenses unless I do myself.

How viable would it be for a third-party alternative to act as a 'broker' between customer and developer, just to handle those aspects you mention?

Steam has done well with that model for games, but being Mac only would be a tough sell. Apple's big advantage was instantly getting the App Store on every Mac.

Who says they have to be Mac only? It'd probably work out better to be on all major platforms, that way you can sell the feature of buy once, run on all platforms the software runs on. You also could allow single platform things onto your store too, like Sketch.

Because the vast vast vast majority of apps aren't cross platform?

So what? Steam sells plenty of games that are Windows only. Doesn't stop them from being available on other platforms.

Not very unless you set yourself up as a store, in which case you've got the problem of trust, both on the developer side (why should we deploy to yet another store), and the customer side (why should we trust your fly by night store).