That only moves the problem outwards; instead of having to keep compatibility within the syscall ABI, you have to keep compatibility within the "base userland library" ABI, which is probably much larger (for instance, printf is not a syscall).
It also moves it to a place where you can have somewhat abstracted types (so chances are the client will work with just a recompile as it picks up the updated typedef), you can much more easily prepare the transition by e.g. using various flags or even trying alternatives beforehand (for instance OSX originally added a stat64 call before before rollbacking that choice and using macros to transition stat to 64b), and you are able to stop before a completely mangled syscall is actually attempted (by checking for some version symbol which must be set if you compiled for the 64b version of the various syscalls).
