Which makes sense when you think of libc as "the library which provides the system call interface to the OS, and just happens to provide a C runtime library as

Which makes sense when you think of libc as "the library which provides the system call interface to the OS, and just happens to provide a C runtime library as well". Making libc dynamic link only allows the OS developers to change the userland/kernel land interface as they see fit without breaking compatibility. The fact that this also ties you to a dynamically linked C runtime library is an artifact of the close ties between Unix and C. Over in Windows land, there isn't a tight coupling between libc and the system call interface. libc is provided by your compiler (Visual C++ provides static and dynamic versions), while the system call interface is provided by ntdll.dll and friends, which are part of the OS and free to change in future OS versions.