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.
If someone wants to implement the C code for a tzinfo concrete class that we are talking about, that's fine. But that will not prevent datetime from getting a pure Python version at some point.