Monospace is weird legacy technology and a path dependency that people are holding on to for not so great reasons.
If monospace was actually a good idea and not a technical limitation of computers from the 80s, books, newspapers, and your comment on this website would be displayed in monospace.
I dunno man, sometimes things are good in different contexts. Monospace makes stuff really easy to align when programming, and while I do like the idea of elastic tabstops and have tried it out before, in practice they don't work so well.
Frankly, I've never desired for things to be aligned in source code. Are you, like, embedding tables of numbers in your code, or just explicitly aligning the equals signs in short = 42 and much_longer = 7?