I agree that the typescript team should consider using the compiler to extend javascript rather than just supplement it with types. Like you said, there are technically parts of typescript that don't have a direct corollary in js (enums/namespaces), so why not take it a step further and add some of the features that js is missing? The only example where they've really taken that next step is with experimental decorators.

I think in general the js/ts community is starting to realize that if we're going to have all of these build tools, we might as well use them to do more than just format, transpile, and bundle. Svelte has sort of pushed this idea with its abuse of the js label feature (it can detect when a variable is reassigned if you prefix it with the dollar sign label, which is technically not possible with js, even with proxies). There's also React Forget, which is promising to completely rewrite components at compile-time to avoid all of the gotchas that hooks have (and improve performance) [1].

In terms of making all of ts erasable, you can technically just write all your type definitions using js-doc comments, in which case the compiler can just be run with the --noEmit flag, and then the code is fully valid js without any modifications.

[1] https://www.youtube.com/watch?v=lGEMwh32soc