I think the answer should be that _eventually_ they should have to migrate, but in the interim we must maintain two build systems.
You are right on the first point, but it is not a requirement for
php-src to be managed by anything other than CMake to maintain phpize
functionality for existing 3rd party software making use of it. From a
quick look, I can tell that it just copies some files that can be
generated/configured by CMake if necessary.
Just so we are in the clear, I'm purposefully abstaining from a deep
dive into php-src's build tooling, because I would really like the video
that I have in mind to be genuine. I had the idea of making a video
about this thanks to Andreas Kling's SerenityOS videos and he follows a
similar process.