[WorldMaker]

TC-39 isn't "betting the house on TS": the proposal is generic and intentionally doesn't define types it defines syntax for annotations of types. It's basically "type information may appear between a colon after an identifier and an equals sign or other terminator" and "type is a statement keyword with type information following it' and "interface is a block keyword with type information inside that block". That's pretty much the entirety of the proposal: it doesn't state what that type information is (just parsing limits on what it could not be so that it doesn't blow up language parsing), what it is used for (other than that the general idea at runtime is that type annotations are comments just like existing jsdoc comments), or who uses it. While the proposal is trying to stay syntax compatible (again, not types compatible because the proposal doesn't include types) with Typescript it also points out existing overlaps with Closure and Flow type syntaxes and is built so that both of those can use the new syntax for their own, different type systems.

This is why it is a Python-inspired proposal: Python didn't define types, Python defined "here's some places in the language that are effectively ignored like comments that type systems are expected to use". Python has made them more than "just" comments in later proposals and TC-39 holds the right to do that for ES/JS in the future but the current proposal stops about where Python's first proposal did at "types are just fancy inline comments for compilers like (but not limited to) Typescript/Closure/Flow".

Again the proposal itself is a really good read for specifics of where it looks like Typescript and what parts of Typescript are intentionally not supported. For instance as a good reminder there are only three (!) features of Typescript left that aren't ES-standardized and generate code: enums, namespaces, and class parameter properties. Of those, both enums and namespaces have been marked deprecated for a long time and many lint rules exist to help eliminate them from your code if you are still using them.