The discuss.python.org experiment has been going on for quite a while, and while the platform is not without its issues, we consider it a success. The Core Development category is busier than python-dev. According to staff, discuss.python.org is much easier to moderate.. If you're following python-dev but not discuss.python.org, you're missing out.
Personally, I think you are focused too narrowly and aren't seeing the forest for the trees. Email protocols were long ago standardized. As a result, people can use any of a large number of applications to read and organize their email. To my knowledge, there is no standardization amongst the various forum tools out there. I'm not suggesting discuss is necessarily better or worse than other (often not open source) forum tools, but each one implements its own walled garden. I'm referring more broadly than just Python, or even Python development, though even within the Python community it's now difficult to manage/monitor all the various discussion sources (email, discuss, GitHub, Stack Overflow, ...) Get off my lawn! ;-) Skip, kinda glad he's retired now...