> If you use a true proportional font, you give up aligning code elements other than basic indentation.
Have you ever gotten deep into how tab stops work in Word?
The deeper you go the more you realize fun things like Tables are as much "Tab Stops with Borders" as they are a separate concept to Word. The UI/UX of both reflect each other.
WYSIWYG word processors and design tools have lots of ways to align proportional fonts.
The big thing is that to do it well they need a ton of metadata: this "paragraph" has tab stops at 1", 2", 4", and 5.5", two of the stops are right-justified and one is centered. Word makes it surprisingly easy to edit all of that metadata easily and visually in the Ruler up top.
If you are sticking to plain text documents that are easy to source control, where and how do you store that metadata? How do you keep it from being a distraction from the code you want to write?
It's not an insurmountable problem, we could do some really cool things if we tried. One half-baked thought off the top of my head here is that I bet you could do something rather cool with easily embedded CSS Grid descriptions in nearby comments and Tab/Newline-delimited sequences auto-populating cells in the grid. Given how much of our code is HTML rendered anyway and how ubiquitous HTML renderers are in our digital lives, CSS Grid isn't the worst model to reuse for something like this, and might be something someone could build a prototype with relatively quickly.