[Retra]

Monospaced fonts are good for editing. Variable width characters are good for quick reading because they allow you to scan past spelling errors in prose. Monospaced fonts have distinctive 'textures' that make them amenable to detecting symbol identity. Variable width fonts intentionally eliminate that texture and are often optimized for ink density to 'grey out' blocks of text. Arrow keys don't work predictably in variable width text.

[runarberg]

There is no reason font designers can’t create variable width font faces that optimize character distinctions and important features while reading code. Many font faces even have settings that allow you to opt into making characters more distinguishable.

You are right about navigating between lines in a variable width fonts. However that is a problem with many text boxes on the web (including slack and github) and I don’t find it that much of a headache. I find it kind of rare that I need to navigate between subsequent lines and maintain the column except maybe at the start of the indentation level.