The C API is already somewhat unstable between releases (X.Y -> X.Y+1), and extensions are supposed to adjust, although we try to keep the API as stable as possible and the cost of changing the API is considered. Anyone who wants to a truly stable API should target the restricted API (aka stable ABI).
In particular the specific layout of most structs is not considered to be part of the API and can be changed between releases (which allowed for the introduction of a new representation for PyUnicode_Type, and allows for adding new fields to the PyTypeObject struct).
BTW. I've subscribed to this list because I'll likely be affected by this. I maintain (and wrote most of) PyObjC which basically is an FFI from Python to Objective-C, and that code base is relying heavily on the Python C-API, including poking at bits that I probably shouldn't poke at. I'm probably less focussed on raw speed than the Cython folks, but still want to keep the overhead of the FFI quite low.