The hazards of 32/64-bit compatibility

Right, but the kernel has to know the layout and structure of all of these structs, anyways, since it has to extract the data to use it. So it's only a little more work, comparatively, to also have 32-bit parse code.

Whereas currently, libc doesn't have to know anything about ioctl formats, it just passes a pointer along. And so if you compile a new kernel module that has some random new ioctl()s, and install the corresponding user programs, everything works. But if libc has to do the marshaling, I also need to update my libc, which is much harder.