I don't see how to square "makes essentially arbitrary syscalls so we can run faster than Linux code" and "is safe". WebAssembly is not Platonically safe; it's safe for specific reasons. If you bind syscalls into it, or more generally, bind anything that allows you to do what syscalls do in an OS, you're going to be poking holes in its safety at a distressing pace, because you're taking some of the reasons WebAssembly is safe, and throwing them away.

It may be memory safe, but honestly, memory-safety is no great trick anymore... pretty much everything except the languages currently used to implement kernels is memory safe. Memory safe is still not "generally safe".