A great list, too. I guess I have stuff to brush up on for the next 10 to 20 years?
I try to (re-)learn math periodically because I feel like I should, like how one ought to eat one's vegetables and one ought to exercise, but extrinsic motivation is the thing that's lacking. I usually end up on recreational math puzzles or something, before dropping it, since at least those are fun. Probably made three or four cracks at it in a decade, each has gone the same way.
I literally don't know what I'd use most of it for—I can't find that need, even when I try. I'm sure I could make something up just to have an excuse to apply what I was learning, but... why? Probably there are other jobs I could find where knowledge of math was absolutely key, but... why? I've been paid to write code for about 23 years, my pay's great, and I've repeatedly gained a reputation at companies for being the guy to go to for tough or low-level problems. But if you gave me an intro to linear algebra final or calculus 1 final, today, I'd be lucky to score 25% on either (hell, I'd be lucky to get anything right on the Calc final, but maybe there'd be a couple easy questions at the beginning—I've never, once, ever applied anything I learned in calc, for any purpose, so what little I knew about it to begin with is long gone)
If all you have is a hammer, everything looks like a nail—but the converse of that is that if you have never seen a hammer, nails will be invisible and incomprehensible to you, they will just blend into the background of noise.
When I learn about something, I suddenly see it everywhere.
Hypothetically, if you wanted to learn it, I would recommend devoting the first hour of every day to it. Before your brain fully wakes up and starts asking why you are doing it, just do it.
I assume you have the standard CS background, with decent knowledge of discrete math, calculus, linear algebra. I would recommend starting with a graduate-level linear algebra textbook. A friend of mine used to say “linear algebra is the new addition”, it permeates everything, and knowing it well produces massive dividends.
> I'm still appalled at how people can manage to gather the courage to utter they don't need math.
When... well, the vast majority of people really don't. They promptly forget almost everything back to about 6th grade math, shortly after finishing formal education, because they truly never need it, so that knowledge and those skills quickly rust.
If these people in-fact could make great use of it, such that it's "appalling" that they don't think they need it, then that's probably what school should focus on teaching, at least for non-math-majors. Laser-focus on application in everyday life. Especially in k-12. If it's actually useful and people are being forced to spend hundreds to thousands of hours learning junior high, high school, and college math, but then losing most of it because they never see any use for it, that's a tremendous failing of curriculum that should be addressed as directly as possible. If such a program wouldn't succeed because it's actually true that most people don't really need most of that math for anything, then we ought not be "appalled" at their correctly assessing that truth.
Personally, if it were “just relearn calculus and move from there” I think I’d be able to motivate myself. At that point I’m still pretty close to the problems and tasks that interest me. But the reality is I spent much of middle school and high school wasting my time doing things other than math, so in reality I’d have to go much further back and relearn all the prerequisite stuff, and the prerequisite stuff to that, etc. By that point Im so divorced from the reason I wanted to try and relearn math in the first places and I just get bored and give up.