> WASI, meanwhile, is just a very small subset of POSIX but with a bunch of stuff renamed so nothing works on it.

WASI fixed well-known flaws in the POSIX API. That's not a bad thing.

> the major downside that no CPU vendor uses it so it has to be JIT compiled at great expense.

WASM was designed to be JIT-compiled into its final form at the speed it is downloaded by a web browser. JS JIT-compilers in modern web browsers are much more complex, often having multiple compilers in tiers so it spends time optimising only the hottest functions.

Outside web browsers, I'd think there are few use-cases where WASM couldn't be AOT-compiled.