what's the argument against this? There are many arguments for how it can improve readability and speed up understanding of what you're looking at when you're not familiar with it.
My opinion on this is that it's a bad idea in the age where we (finally) have near universal acceptance of source control, because they bloat commits with irrelevant changes in surrounding lines to preserve alignment when identifiers change. Black got it exactly right - optimize for readability and change locality.
If you feel like it’s useful, you should probably be using an editor with real elastic tabstops instead.
You picked the wrong vocation my friend. It's "brittle and fiddly" all the way down.
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?
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.