the fact that most editors use mono-space fonts by default and hardly anyone is clamoring for the ability to use proprotional fonts seems to argue against them being particularly important to readability, at least as far as the community seems to be concerned.
re "you should probably be using an editor with real elastic tabstops" ... i don't get what you're trying to say. What does real or fake elastic tabstops have to do with the choice to align code beyond just indentation?
Manual alignment is often used as a poor man’s elastic tabstops.
I think there’s a chicken-and-egg situation with monospaced type and programming. Programming was a thing before digital proportional typefaces were a thing, so some people came to rely on the fixed spacing to treat their source code as a 2d grid rather than a 1d buffer. This in turn made it more difficult to switch over to proportional typefaces despite their advantages, and at some point code == monospaced was just taken for granted. Another issue that comes from mistaking your code for a 2d mosaic is the dogma of dictating a maximum number of characters per line, instead of the more reasonable approach of using whatever expresses the structure of the program most clearly, and relying on line wrapping to make sure it’s all visible.
FWIW I use proportional fonts, and 95% of the time it’s completely unproblematic. Fortunately I rarely work with code that uses manual alignment. It’s usually the occasional ASCII diagram that forces me to temporarily switch over to fixed pitch.
re "you should probably be using an editor with real elastic tabstops" ... i don't get what you're trying to say. What does real or fake elastic tabstops have to do with the choice to align code beyond just indentation?