Something 'deeper' than Emacs, or am I looking for a unicorn?

You could dig deeper into software archeology and go with TECO, the editor that EMACS was first written in. It would be silly to do so, however.

> Stronger expected lifespan

I think Emacs is a good example of the Lindy Effect. It's going to continue being alive for a long time because it's already been alive for a long time.

https://en.wikipedia.org/wiki/Lindy_effect

The only editor that I can think of and matches Emacs on any of those points is Lem [1]. It has a few hypothetical advantages over Emacs:

- Lem is written in Common Lisp. It's less of a niche language than ELisp. You are therefore likely to find a larger library ecosystem.

- RMS didn't really like CL when he wrote Emacs. But CL is arguably a much better language today. For example, you don't need to worry about dynamic scoping.

- Lem is written entirely in CL, without any C core like Emacs does. That possibly makes Lem customizable to a deeper level than Emacs.

- Being a very young project, Lem is likely more optimized for multithreading compared to Emacs. Emacs multithreading is not up to expectations, perhaps due to legacy plugins and code.

Lem is not capable of competing with Emacs on any other points on account of the huge difference in their ages. Lem has much fewer extensions and a much smaller community. I don't know the project well enough to comment about their longetivity. But it's worth a look.

[1] https://lem-project.github.io/

If you've been on a long journey of trying different apps, you may just be restless now that you've found a tool that mostly works. Stick with emacs, use your extremely limited remaining time on earth to do awesome things.

Emacs is more than enough, you do need more than 1 app.

Customizability: what are you trying to customize but you can’t?

Packages: https://melpa.org/#/

Lifespan: Emacs hails from the 70s and is actively maintained.

There are literally 100s of text editors, so it really is more a matter of taste and preferences. However, to the best of my knowledge, emacs with eLisp is unlikely to be eclipsed on any of your points.