So to avoid re-opening the debate of Python 2 vs Python 3, let's take this point of view: The Python 3 language may, or may not, have been an improvement, and the transition from 2 to 3 may, or may not, have been done correctly. Everyone has its own opinion on this. But at the end this problem concerns the Python community.

From the Arch developers point of view, the Python developers decided to release a new version of their language. They were going to support python 2 for a while, but the direction that the language was taking for the future was Python 3, so one day or another Python 3 would become the standard, so doing the switch was the good decision.

As said by @tbranyen the work of adapting shebangs have been done on the Arch side anyway, so I don't see a problem for people developing their projects in Python. Also Arch users are mostly power users (you need to understand quite a bit about linux to install it) so they are usually able to handle python 2 vs 3 errors quite well, especially as it is known than Arch use Python 3 by default.