Yes backward compatibility is not an option (lets avoid redoing python3 trauma ) and simply delegating coexistence of old and new features to implementation is a bit lazy. However documenting one way as the current one and other features as old ways, for backward compatibility can simplify things.
We're not done with Python 2 yet. This would be a Python 4 for sure. That's not according the philosophy of backward compatibility of Python (even if I agree with you).