> and expecting them to be skeuomorphic to a particular approach may hold us back
Whatever a programming language is, it has to be something you can hold onto in your head, because the human mind is where "programming" takes place. And there really aren't many forms that such a thing can take.
The step-sibling of programming languages, the mathematical proof, can really take any form we like; we're not limited to maths that can run on a computer, as long as we can "run" them in our minds. But we've still ended up with only two basic formalisms that almost(?) all mathematics is expressed in terms of: geometry (visual mathematics), and algebra (symbolic mathematics, including formal logic.) Because those are the formalisms that we have mental hardware to comprehend.
Anything that isn't in one of those shapes (say, an exhaustive computer-assisted proof-by-parts) might still be productive to prove; but we humans won't like it, and will always seek to replace it with something that we can get a mental "handle" on.
We'd certainly never take the formalisms that do work for us, throw them out, and replace them with non-human-mind-comprehensible formalisms instead. Why would we bother? What would we gain?
> Being able to push secondary parts of a program to be foldable, or visible on mouseover but not by default, makes it easier to communicate understanding to other humans. And actually that's a more faithful recreation on how you'd communicate in writing - on a blackboard you might use small text, or footnotes, or a verbal explanation.
Oh, I don't disagree. But all those things are useful for plain text, too. As you say, humans already think and manipulate text that way. And "advanced" text editors already provide facilities to manipulate text that way, without any understanding of programming-language syntax.
You can get very fancy about how you manipulate symbols... as long as "just manipulating symbols" is truly all you're doing. As pure symbol manipulation is something you can always also do in your head. Go much beyond that, though, and you stray from the land of "things humans can keep track of in their brains."
Can you think in terms of what a JetBrains codebase will look like after you refactor it, without actually doing the refactoring and seeing the result? How about with two or three such applications in play? It gets pretty hard, no? Because those aren't just pure symbol-manip. They're not just moving letters around on the mental blackboard that represents the current module. The non-local effects aren't strictly intuitive. Each layer of that that you have to hold solely in your head, bogs you thinking down. In a high-friction language, you just don't bother to try to work too far ahead with such changes; you just "take things one step at a time", applying each change and then re-learning the codebase in light of it; sometimes applying a change and then backing out once you realize you're got your sequencing wrong.
Sometimes working "with blinders on" like this is necessary (e.g. fixing up a big ugly three-way git merge); but why subject yourself to it if you don't have to? Wouldn't you rather be able to understand what you're doing, and where you're going?
> A mathematical paper is almost never "plain text" in the sense of being a linear sequence of ascii characters and nothing more.
I mean, they're usually written in LaTeX ;) But to be clear and precise, I was trying to invoke the concept of a https://en.wikipedia.org/wiki/Formal_language. Programming languages and (symbolic) mathematics are both formal languages. You manipulate them algebraically, by moving the symbols around, applying transformations that turn one valid statement into another valid statement.
Yes, there are isomorphisms, e.g. visual "programming languages" like Scratch. They're certainly neat, but as far as I've been able to ascertain, people can't really think in them the way they can think in symbols.
Ask a composer: what do you think about when you compose music? Do you think of the sound; or do you picture the score? Where music's concerned, I would guess pretty much everyone is thinking about the sound, because that's a lot more intuitive. Someone could probably write a song entirely by manipulating a score in their mind—and make it sound good!—but it'd be hell to do, in comparison.