> There is no alternative.
There have been several projects like Ubuntu Touch to create an open Linux for smartphones.
That would be an open alternative.
Android is not open.
The AOSP is fully open source. What you suggest is a significant step back in terms of privacy and security.
Android is an open source Linux distribution for smartphones. It doesn't have to be used with Google Mobile Services. AOSP is far more private, secure, usable and compatible with a far broader amount of apps. Why wouldn't we use that as the starting point even if the goal was to diverge?