TeX has the idea that it's feature complete and should not develop, so then by definition (cheekily) it cannot attract any developers.

With all due respect to your perspective on TeX, isn't it the whole point? There are weaknesses in TeX (like using $ for both beginning and ending the math mode) but they are quite minor. Quirkiness is not a weakness but being a standard (since 1986!) is a major strength. The fact that it does not change is a blessing, truly. Python may be a fine language with a great community of supporters and I use it because I pretty much must but programming in something that is essentially a moving target is no fun. And I would take TeX macros over Python ugly syntax any day, no offense. Not to say I am right just to point out that taste is not necessarily a reliable guide.

The modernized version is Typst though.. it's closing the gap to (La)TeX really fast