This proposal is essentially a way to standardize a subset of typescript (a subset for now...).

It's Microsoft imposing their technology on the ECMA committee.

Ironically the same Microsoft that rejected ES4. We wouldn't be needing Typescript if ES4 was adopted 15 years ago...

edit: While this proposal doesn't have runtime typechecks, it's just one step toward adding runtime typechecks it isn't a stretch to think that if this passes, runtime typechecks might eventually be added to the language. ActionScript 3/JScript.net/ES4 had them, so it's perfectly possible for Javascript "with types" to add them as well.