The only breakage I can see in this proposal is that the [a-z] dns
names go away. That would take four months. I think perhaps that's a
bit short. I don't see why we can't keep them around for much longer.
I think we're all willing to increase the time :) The current timeframe was somewhat arbitrarily suggested by Noah and I just rolled with it for a first draft of the PEP figuring if it was too short someone would (hopefully!) speak up.
The other breakage is people relying on --use-mirrors in PyPI but that is NOPd in the upcoming pip 1.5, and the side effect of removing these names in older versions of pip simply won't get mirroring support (It's mirroring support still hits PyPI itself).
A way to find mirrors is needed, but perhaps not automatic, but for
when pypi goes down.
Thank you for calling this out, I forgot to include in the PEP that moving the mirroring listing off of PyPI itself means that we increase the chances that the listing will be available if PyPI itself happens to be down.