I'm looking for software that is powerful, understandable, hackable, extensible, reliable, future-proof (few breaking updates) and feature-rich.
The two tools I've found so far that come close to this are TiddlyWiki (notetaking) and Vim/Emacs (same text editor niche). TiddlyWiki loses some points on hackability because it's web technology and fairly sandboxed. Vim/Emacs are only for text, obviously. I think what these tools have in common is that they all have a strong core idea (Vim: modal, TiddlyWiki: a quine of tiddlers, Emacs: elisp) that provides insane feature breadth at relatively low cost to learnability.
Other software is usually on a spectrum of low feature count (most tools in the UNIX philosophy) to low understandability (IDEs, most other text editors, all OSes). Then a few rare ones are extremely powerful, but so old and poorly designed that I don't think they make the cut (e.g. Bash in my opinion).
Are there other tools similar to Vim, that maybe even cover other application areas? Doesn't have to be terminal-based. I'd love to hear your opinions!
I don't know if I missed something in your post, but this couldn't be further from the truth. It's cliche to refer to emacs as it's own OS, and it is not uncommon for users to do all of their computing inside of emacs - web browsing, email, note taking, documentation, writing, version control, terminal, ide, window manager, remote admin, etc. Based on your criteria emacs seems like a perfect fit, I encourage you take take a deeper look at emacs if you're interested.
https://github.com/emacs-tw/awesome-emacs