> We don't maintain any other features in two languages for those
purposes. IMO, it will just be more of a burden to fix bugs in two
different places as compared to the advantages you mention.

Surely the majority of the burden is imposed by the C implementation.  I
expect that 90% of the time spent fixing bugs will be spent fixing them
in C.  So for only a slightly increased maintenance cost, a massive
advantage is gained for other Python implementations.  If the general
well-being and popularity of Python isn't a concern of CPython
developers, then perhaps the benefits can still be preserved at minimal
cost to the CPython developers by letting some Jython, IronPython, or
PyPy developers maintain the Python implementation of the io library in
the CPython source tree (rather than making them copy it elsewhere where
it will more frequently get out of sync, and where
Jython/IronPython/PyPy might waste effort in duplicating maintenance).

Or maybe none of them will care or object to the removal of the Python
version from CPython.  It might at least be worth asking first, though.