Sorry if these are repetitive questions btw. I'm just curious.
The kernel already has moved pretty far forward. BeOS audio drivers, for instance, don't really work anymore in favor of our new multi-audio API (I don't think anyone had any use for BeOS audio drivers; we merged all the open-source ones, I think), the VFS layer has gotten major upgrades, we rewrote the thread scheduler and removed the 8-core limit we inherited from BeOS, and already have added ASLR and DEP support, among lots of other improvements. I think the only thing at this point which actually is still compatible kernel-wise are FS drivers (sorta) and graphics drivers (although of these, we have more in-tree than BeOS ever did, so I don't know if anyone even uses old BeOS ones anymore).