Monospace fonts are used mainly because vertical alignment is desirable, and because characters such as ! and . are too narrow to easily read in proportional fonts. Monospace, then, improves clarity. Turning a => into a ⇒ does not affect the vertical alignment nor make anything too narrow to read. It just turns a combination of symbols into its semantic meaning. It allows you to parse a meaningful glyph rather than using a hacky workaround to represent meaning via ascii. This is also the point of emojis: (-_-) is fine, but most people find <expressionless face emoji> to be more clear as to the meaning. Nobody is getting confused about what these ligatures mean. They installed the fonts and selected the fonts in their IDE; they have already been coding; they are not going to see ⇒ where a => used to be and have a brain meltdown.