For instance, in this very convo what matters is that we're talking past each other, and I think that'd likely happen regardless of whether our comments were in fixed or proportional fonts, regardless of what other typographical choices might be made.
Well, you presented your use case as an "on the other hand" counter argument for me saying what sort of fonts generally work better. Which it isn't, it is just a specific use case where monospaced fonts happen to be used and it is difficult to change them.
That doesn't change the fact that in environments where there is control over all that monospaced fonts rarely are the best choice for large bodies of text.
For instance, in this very convo what matters is that we're talking past each other, and I think that'd likely happen regardless of whether our comments were in fixed or proportional fonts, regardless of what other typographical choices might be made.