Wait, where is the HPy project in that plan? :-) The HPy project (brand new C API) is a good solution for the long term!
My concerns about HPy right now is that, in short, CPython has to continue supporting the C API for a few more years, and we cannot evolve CPython before it will become reasonable to consider removing the "legacy" C API.
I explained that in details in the PEP 674 (Disallow using Py_TYPE() and Py_SIZE() macros as l-values): https://www.python.org/dev/peps/pep-0674/#relationship-with-the-hpy-project
In parallel, we should continue promoting the usage of Cython, cffi, pybind11 and HPy, rather than using directly the C API.