I think kernel modules will go away at some point. Having no third party Kexts would increase the security of the OS for in use systems. That's a nice way of saying not all third-party Kexts are created equal.
I could see an argument where moving existing hardware Kexts to user space is easier because IOKit uses the libkern C++ Runtime. The OO design of IOKit may lend itself very nicely to the driver approach BarrelFish takes (http://www.barrelfish.org). The real hard one to move to user space would be third-party filesystems. That's mainly because of dated VFS architecture used in *NIX systems. I could see Apple completely moving away from that at a future point too.
For example USB to Serial devices, or custom media devices, and more. I really don't expect kernel modules to go away.