Apple, Google, and... ?

If you include Steam, we need to include XBox, Nintendo, and Playstation too; I would expect they're not too different in size.

But yes, the status quo needs shaking up unless we really want to live in digital neofeudalism.

Microsoft? They have a billion users.

Or maybe Amazon which has a decent sized Android app store.

Do people really download anything from the MS store...? I got Python from there once and it's been nothing but annoyances. I guess I'm technically one of that billion users, but I'll never download anything from there ever again.