The benefit is that we can forgo any build or compile step, and just ship code as is. There will be no source maps error to worry about. We will be able to run and debug the exact code that we wrote and not some jumbled transpiled code. We can just have a local type checker that can help with correctness and inline suggestions in our ide.

When I say nothing will be locked down, then that does only mean the type checking itself will not be locked down. Indeed it will not be specified at all. Buuuuut the location of the types in the js syntaxt will 100% be locked down and specified. That's what the proposal is. So there will be limitations on coming up with novel ways to integrate type syntax with the js syntax. But you will of course still be able to make your own compiler if you want this.