I know this is orthoganal to your point, but Arch Linux ships with python3 installed by default. Of course, python 2.7 is in the repositories, but you have to say /usr/bin/python2 to get to it. That breaks all those stupid build scripts that expect python 2.x to be installed in /usr/bin/python and nowhere else.
Wait... it's stupid to write a script that uses /usr/bin/python (or really "/bin/env python" per the convention in that community)? When those scripts were written, there was no "python2". It's certainly not their fault that "python" changed to be an interpreter for a different language.
This is exactly the problem. Distros (c.f. Red Hat, SUSE, Ubuntu) with large installed bases simply cannot ever ship a /usr/bin/python as anything but python 2 for exactly the same reason that the kernel cannot change syscall numbers. It will break their customer's software. Arch Linux doesn't really have "customers" in this sense, so they're free to play. The serious distros aren't.
Just mark my words: python 3 will never be a default "python". We're entering year 4 of the python3 era. You really think anyone's going to jump now if they haven't already?
