I find this a really elegant approach.
While the SC's decision to keep the syntax uniform is certainly laudable, it's creating the issue of packaging new complexities into a very limited syntactic & semantic space (e.g. no new magic symbols like "->", which I agree with BTW), leaving only very verbose solutions that the typing crowd is chafing against.
I think accepting that typing has a syntactic cost on the python language as a whole is unavoidable at some point (and I'm not saying that's a bad thing). Having a separate & opt-in mini-language for type declarations seems like a really clean way to delineate that cost resp. extension, and I especially like the t''-string syntax.