Pretty sure you didn't read (or maybe just missed the point of) what I wrote. Windows is closed source, but it is 'open source' if you work at Microsoft. It is also 'open source' if they give you source code access during an audit or something like that.

Right now, Android is based on the AOSP. You or I can go to the AOSP site, download the code, and do stuff with it. That's a 'true' open source project. Google COULD develop it completely behind closed doors, even with partners, and not release the code to the public. They could still, however, give the code to device manufacturers for free and they could use it in much the same way they do right now. That's not 'open source' in the traditional sense, it's just collaborative closed source software, like how Apple's CarPlay software is worked on collaboratively with the manufacturers of the cars the software runs in. Yeah, the car manufacturers get to see some source code (probably), but no, it is not 'open source'.

And user kernels/ROMs have basically zero impact on Android's global success. They are significant for the communities they serve, but they do not influence the market share of Android. If someone can use Cyanogenmod, they already were using an Android device, so it doesn't make the market share go up to have more custom kernels or 'distros' out there.

Again, I am not saying that Android SHOULD be closed source or whatever. I am just saying that its open source nature has very little to do with its success. It succeeded because it is on a shitload of devices, and that can be accomplished with or without being open source. Windows is everywhere on desktops/laptops because it runs on almost anything, and it is closed source.