Also, are browser text area inputs monospaced by default for everyone? Or did I configure that for myself long ago and forget? If it's not just me, maybe the "reasons" you're alluding to are not so obvious. Anyway, I have no trouble at all reading the long comments I type into text areas.
And more power to people for embracing agency :)
You underestimate the number of HN users who are reading this site in their terminal. ;-)
Legacy decisions as a remnant from a time when taking more space on paper cost pages and therefore resources, remaining as a default from centuries of inertia in how text is printed?
But seriously: you do you. There are people who code in proportional typefaces and they're as baffling to me as you are right now. Let a thousand Markdown viewers bloom.
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.