Brett Cannon wrote in a comment (msg106498) on another issue:

"""
The stated long-term goal of the stdlib is to minimize the C extension modules to only those that have to be written in C (modules can still have performance enhancing extension back-ends). Since datetime does not meet that requirement it's not a matter of "if" but "when" datetime will get a pure Python version and use the extension code only for performance.
"""

I'll keep this open for a while to give Brett and others a chance to respond to opposition.