Speaking as a former C developer, why do "We need to let the community develop their own extensions"?  What's insufficient about Python's current extensibility?

The complexity of a language varies with the square of its feature count, and adding macros intended for creation of domain-specific language features  balloons the feature count. Granted, they don't much increase the complexity of CPython, the language implementation, but the de facto language then becomes more than CPython - potentially a lot more.

That is, making it easier to extend python means a proliferation of extensions with little thought given to their long term viability or cross-domain compatibility.

It's kind of like zsh vs. bash.  zsh has a smaller implementation, but a larger _language_.  For this reason, I'm not terribly interested in zsh, but I like bash.  On the other hand, bash has seen a proliferation of unnecessary extensions in recent years, so I may jump ship to something else someday - something with a smaller language, that isn't afraid of fork+exec.

I've used m4 before as a macro system for Python+Cython, but I would never consider suggesting that m4 should become part of Python itself.

IMO Perl is dying because of its exuberant design.  One of the most important things a language designer has to do is say "no" sometimes.