You can fork AOSP, which I'm sure can't run any of the top 10 apps of the Play Store unless you install Google Services, which are closed source and require a licence from Google (which in turn requires you to bundle Chrome, make Google Search the default, etc).

AOSP by itself is pretty useless because it can't run any of the apps the public is interested in.

You can't freely fork the Android that's dominating the market: Android with Google services in it, key components of it are proprietary. You can't even sell some devices with it and others with an AOSP fork.

That sounds like a volume discount, which is actually encouraged under most capitalist models to capture consumer surplus. Its not exactly what happened. To put it simply, Microsoft used a complicated pricing model where the OEM paid a lump-sum to Microsoft based on expected sales and Microsoft applied zero marginal pricing so every copy of windows was "free". But if they exceeded their sales they then incurred a fee for every additional unit sold. If they increased their sales, then the lump-sum would have to be renegotiated for the next contract. They wanted to tax the 'output' of the OEMs instead of just selling them at volume. It was a form of predatory pricing which got them into trouble. I don't believe competing OSes were ever a big threat to Microsoft.