It feels like this sort of syntax convention could be used to bolt on all sorts of interesting features to the language. Like, at this point, this is basically a sort of compiler hack to add features rather than really do any typechecking.
It's based on an ECMAScript proposal that is going to be added to mainstream javascript. Typescript is just adding it faster before the browsers.
Stage 3 is "implement it and provide feedback based on real implementations" so in some respect Typescript is adding it at the same time as browsers (and is one of the parallel implementations expected to provide feedback ahead of Stage 4).
By coming up with even more special-case syntaxes to replace "const"? e.g. you want to have something like "foo x = something.else" to replicate the syntax convention of "using x = ..."? I'm not sure I understand what you're referring to.