Does TS has pattern matching?
This trade off is good because there is no runtime overhead, only desugaring. Typescript should never become ML or anything else that Javascript is not. If you want pattern matching in Typescript you need to convince the Ecma committee to put pattern matching into Javascript first.
What I don't see is how this requires the addition of a "runtime" on top of the JS model to make it work. I don't use TypeScript, and don't know how it implements its data types, so there may be something obvious here that I'm missing.
But it could use JavaScripts own types.
But it couldn't use TypeScripts types.