I kind of think monospacing is overrated in programming. Sroustrup typeset the entire of The C++ Programming Language in variable width font and it looks totally fine. I've used variable width fonts in my editor a fair bit too.

I think the main problem with it is you need to use tabs for indentation and unfortunately spaces comprehensively won that battle. IMO that's because although tabs are clearly better, spaces are definitely more idiot-friendly and there are a lot of idiots in the world - or at least people who don't give a shit about nice formatting. So large tab based codebases tended to end up with a horrible mix of tabs and spaces.