Ligatures caught on because users like 'clean' designs where 'clean' means 'the removal of affordances which are not needed by frequent users but are useful to new users'. Your example doesn't make any sense because while the dash arrow attempts to mimic an actual arrow it's not ambiguous that it's two characters and will be understood by a compiler as such. A reading of source code either on a blog or shoulder surfing with this font does have that ambiguity, which is the problem.