WebAssembly has a few more advantages when deployed as part of the kernel; you can run things in ring 0. Even better, you can transparently remap things in memory. And even better than, you can realize IPC as a simple function pointer. Safely.

Of course, you can't have everything as WebAssembly, some core drivers will need to run some critical machine code, but those could be tightly enough integrated that the overhead is almost zero (ie, by using WA imports you can turn this into a function call overhead)