Isn't most of the point of WASM to enable portable executables? Why wrap your portable bytecode with an entirely non-portable platform API? Seems mad to me.
To play devil's advocate: targeting WASIX still gives you the benefit of compiling existing code once and running on any OS-architecture pair that Wasmer supports. I long for this sort of thing when building software that needs to run on Linux on multiple architectures inside AWS and also on developers' MacBooks. (Ever tried cross-compiling to macOS? Yeah, I prefer pulling teeth, too.)
Caveat: you'd need to use Wasmer forks of, e.g., rustc and Tokio, but I imagine there are at least some people who would be okay with that.