You're talking about kernel vs userland. That's not base vs non-base. The BSD distributions unintentionally muddy the waters by calling what is an equivalent of a linux distribution a base system and so lose out on the distinction you are drawing.
Drawing the line at the kernel syscall ABI is arbitrary; no other major operating system other than Linux does so.
Consider how difficult it is to ship 3rd-party applications for Linux and all its myriad of distributions; the only thing you can truly rely on is the anemic syscall ABI provided by the kernel (vs. userland).
So I don't really know what you're getting at. If you're not talking about kernel vs userland and you're talking instead about ABI compatibility then the ABI is even more stable and requires even less work to port any piece of software from one distribution to another.