I don't see this ever happening because Django has no dependencies on any other Python libraries. It wouldn't make sense to start requiring SQLAlchemy only to reimplement the existing ORM on top of it for little to no benefit.
I see what you mean, but arguably those aren't really dependencies if they are included in the Django tree. Also, it's definitely not something that would work with a large library like SQLAlchemy.
I think the packaging and setup tools are getting better so maybe it doesn't always have to be that way.