I agree that instead of implementing the Linux syscalls implementing just those libc functions that are OS dependent is sufficient for a large fraction of the existing programs.

However, this means implementing a POSIX layer, similarly to Windows NT. So it is a solution more distant from a "non-POSIX operating system".

While a large part of the Linux syscalls have been motivated by the necessities of implementing POSIX, there are also significant differences and the Linux applications that require the best performance must deviate significantly from POSIX, so they are no longer based on libc for I/O.