So we agree that for 99% of programs, the tool I'm asking for makes sense? I'm OK with that.
I disagree that you need to move the portability layer in sync. It makes pragmatic sense to have first-class and second-class platforms. First-class platforms can be in sync, second-class platforms can be a bit behind. As a user of a second-class platform, I understand this can be a pain, but I've also seen how it ultimately ends up work out.
At least with the portability layer, you've limited the number of lines of code that a porter needs to focus on.
 Gentoo. I'm sure the various BSD groups have it worse, but try running a pure-stable Gentoo. You pretty much can't. And there's no official stance on whether running a mixed stable/unstable setup is supported; ask two developers, you'll get three answers.