The problem that Poeterring tried to address with things like systemd is very obvious when you fire up emacs.

There's so, so much old code. There's very few tests because it was all written hacker style. Because of this "hackable to the core" philosophy these codebases are piles of unmaintainable tech debt. Someone had an itch to scratch, wrote code to scratch it, then submitted a diff for merging. But nobody took a step back and said: "Wait! We need to rearchitect <this> to accommodate this new functionality." This is understandable because GNU projects are volunteer projects and it's a lot more fun to submit a diff than do a refactor to get a small change in.

Old Unix init systems, much like Emacs, were just piles and piles of kludges that kept things going. The insight that Poeterring had was that if you reduce the hacking surface to mere configuration then you only need to maintain the configuration layer and not a huge kaleidoscope of functionality.

The modern Unix layer of having opinionated, config-driven tools talk over pipes won out in this environment because you only really need to maintain the contract at the pipes (even if you are just outputting terminal escape codes.) Beyond that tools can be composed, but the surface area of each tool is small. This is why vim is so much more commonly used and easily maintained than emacs.

(This is distinct from the "Unix philosophy" stuff that was popular before where folks wrote small, composable tools instead of larger, monolithic, config driven ones.)

A more modern approach of "hackable to the core" I feel would be to standardize around a sturdy, opinionated, well maintained core and then to allow extensibility around this core. Helix is exploring this (using Scheme as its extensibility layer.) But I think Helix and other projects (maybe Lem?) need to prove this approach out before it can come close to challenging the modern configuration-driven ecosystem of tools.

(I say this as a user of emacs and Zed by the way and someone who recently started hacking on Lem, so this comes from some experience.)