(Replying to myself.) For what it's worth, I somewhat regret the wording of the parent post. I was hoping for a thread on lessons from Emacs generally applicable to the topic of malleable systems — maybe things like: (1) periodically integrate the community's best "packages" into the "core" else you'll have a mess (also consider: LaTeX), (2) figure out how you can continue to change defaults without fear of breaking old users' workflows — whatever, I was hoping for more lessons — but instead it (I should have known this!) became a thread mostly about Emacs in particular.
