It wouldn't be very useful, since a TS-in-JS runtime would have significant performance overhead, and a TS-in-WASM runtime would have very expensive JS interop

It wouldn't be very useful, since a TS-in-JS runtime would have significant performance overhead, and a TS-in-WASM runtime would have very expensive JS interop plus cross-language-GC hurdles. Might be less bad with WASM GC.