The goal of typescript is to provide static type analysis at compile time and mimic Ecmascript spec features. If Typescript starts bundling a runtime with it to do things completely unrelated to Javascript, then it's not just javascript with types anymore, it becomes its own language which defeats the entire purpose of Typescript.

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.