Right, much like HTML pages indicate what "dialect" they are talking, the python interpreter could understand such hints and do what is expected from it.
That makes much more sense then having to choose what /usr/bin/python is, and whether the new interpreter needs to be /usr/bin/python3 or not.
I am sure the Python developers thought about it and rejected it for some reason, but it seems the most practical way for a distribution or a programmer to migrate gradually their whole stack.