It feels a little bit like if you'd want a linter to be part of an ES spec. As in: I appreciate TypeScript for the compile-time guarantees it gives me.
A good comparison here is the Python approach: Python 3 made the syntax for type annotations a first-class part of the language but only to the point of parsing that syntax and providing an AST, it did not (and at least for now will not) define "compile time" or "run time" meanings for them.
The immediate benefit to adding Typescript/Flow-like annotations to the ES spec would be that you could run Typescript files unchanged (without transpilation) in the browser, even if the Browser didn't do any type checks for you. You could still use TS or Flow to do the type checks in a separate process, but you could potentially drop the type-stripping steps in TS or Babel. The possibility exists that eventually the browser could also start to enforce basic type checks, but the immediate benefit of slightly faster build processes with no type-stripping step shouldn't be overlooked.
That makes sense. I can also imagine not wanting to start too quickly with that though, to learn what such a syntax would have to support from Flow and TypeScript.
The point of having type annotations in the spec is so that library and tooling authors can focus on providing and refining a single set of type annotations that any tool (Flow, TypeScript, anything else that comes up) can check for correctness and perform static analysis (for things like autocomplete, find references, go-to definition, etc) against. Over time, this would vastly improve the quantity and quality of available type definitions compared to what's available for either the Flow or TypeScript camp alone.
And once type annotations are part of the language proper, the possibility opens up for browsers to use type annotations to provide runtime safeguards, optimizations, and reflections, all of which would be invaluable since static checking alone is often not enough to provide correctness guarantees in all cases in JavaScript. When all is said and done, JavaScript is a dynamic language, and the ability to reuse the same type annotations for both static and dynamic checks is the piece that I'm sorely missing from both the Flow and TypeScript ecosystems so far.
The immediate benefit to adding Typescript/Flow-like annotations to the ES spec would be that you could run Typescript files unchanged (without transpilation) in the browser, even if the Browser didn't do any type checks for you. You could still use TS or Flow to do the type checks in a separate process, but you could potentially drop the type-stripping steps in TS or Babel. The possibility exists that eventually the browser could also start to enforce basic type checks, but the immediate benefit of slightly faster build processes with no type-stripping step shouldn't be overlooked.