> The "legacy" is monospace! We went way out of our way to to get proportional typesetting working.

The legacy is proportional, at least in Latin script and its ancestors. Handwriting was proportional, of course, and so was Gutenberg's printing press. Books and newspapers have virtually always been printed in proportional type.

In Chinese and Japanese, monospace is legacy in both handwriting and print... and also still universally used today. All Chinese and Japanese text is monospaced by default. Billions of people are getting by just fine reading monospaced prose.

I don't really know where this conception that monospaced is somehow objectively harder to read is coming from. Actually, this is the first I've ever heard of the complaint. I can't help but wonder if you've been subjected to some very bad monospaced fonts in prose or something.

Monospace text is objectively less dense, which means you have to move your eyes more. Every eye movement is an opportunity for error. Monospace text only makes sense when seeing exact character counts matters (which it often does in computer code).