Yea this PEP makes sense to me.
The only thing I was able to find that could be improved in my view, is the name “solid base“. It feels a bit general and non-descriptive to me. I realize that the name has some history in cpython. But if we can think of a better name, then I see no reason not to 
Some alternatives for “solid“ that come to mind are “singular“, “prime“, “disjoint“, “exclusive“, “closed“, “mono“, and “solo“.
So unless the “solid base” is set in stone already (which would be pretty ironic), then maybe a poll could be a fun way to decide?