Note, it is not that it renders documents well. While, I do appreciate that fact, the real strength is I am not constantly having to play gardener with all of the documents I've written. Any bitrot in my document will come from facts in it, not because this year's version decided that promises were better than callbacks. Or that "box-sizing: border-box" is the more natural way to do things. (Or yet another brand new construct to lay things out.)
I think a lot of this stability can be traced directly to Knuth. He has a discipline that is basically unmatched.
And it isn't like he hasn't had some misgivings about choices he has made. However, his main choice that he is willing to stick with is stability. And that is huge for long term use.
[1] https://github.com/latex3/latex3
[2] https://www.latex-project.org/help/documentation/ltx3info.pd...
As other people have mentioned, the LuaTeX folks have got a real good thing going. It is climbing the adoption curve but it makes reasonable some things that in the past have not really been reasonable to program (at least for me).
I understand the limitations and design decisions made at the time, but I think it's time for something new. Maybe just using TeX as a backend language would be acceptable, though I think it would just be preferable to start anew. I know there's Scribble but I don't think it handles equations.