There’s an extremely long thread about this from when this was first discussed, and the main reason is for backwards compatibility and the ability to undo it if the community decides they don’t like it.
Doing it inline would basically require it to be a 2.0 bump and make it difficult to “try it out” or reverse course.
With the current approach, once type annotations are fully supported, you could implement your own inline approach and if people liked it, adopt it, and maybe the language could evolve that way anyway, but there’d be more flexibility to try different things out along the way
