This is not a software question. This is a business question. Achieving this would be nearly impossible. It's not a technical problem. It's related to network effects. Alibaba tried and failed.
-
The business challenges associated with a mobile OS are many times greater than the technical.
I disagreed with your second sentence. I didn't make that clear. I don't know if they don't have a choice. I don't know enough to answer the question. I imagine that part of Android will remain open source and usable if they need it.