> In fact the advantage of moving libc into the kernel is that we would NOT need to be so worried about the ABI between libc and the kernel since they were modified in sync.
Not unless you want to eliminate static linking and make upgrading your kernel a nightmare. (If the ABI the kernel exposes to libc isn't stable, then libc has to be updated with the kernel -- except that the kernel update isn't effective until reboot, while the libc update will be effective for newly launched programs immediately.)
Solaris has this problem, and "solves" it via a horrible hack called "deferred activation patching" -- basically bind mounting the old libraries over the new ones until reboot. (It's also why there's no static linking on Solaris >= 10.)