Why didn't the Python committee opt for a compiled system (like PyPy) when they moved to the 3.0 series (and had to break backward compatibility anyway)?
You are right, i think that Python is trying to be as expressive and succinct as possible. A runtime like pypy is very difficult to change, and it would therefore make it much more difficult to evolve the language.
Oh I see what you mean, I misread the sentence. Switching between language backends has nothing to do with compatibility. Yup makes sense. They can swap implementations at any time.
The point really is that PyPy has some compatibility issues with the C api I think mostly because of the garbage collector. This has less to do with whether you compile or interpret bytecode, yes.