Personally I can't see why they don't stick an IE badge on webkit and be done with it.
OSX is also modeled like this. There is a Mach kernel and a BSD-like personality on top of it.
Wine allows Win32 software to run on top of Unix without ever realizing they are running on top of a "fake" Win32 stack.
I think GNUStep could be used to build something similar that could allow OSX software to run on non-OSX hosts.
Also the browser is taking up more of the OS capabilities, like threads, storage, GPU acceleration and so on. So MS would be reluctant to let the browser become too strong. But the horse has already bolted and there's always Linux, Android, Meego etc for the browser makers to cater for the netbook, tablet, smart phone markets.