It's pretty well known that monospaced fonts are harder for reading prose.[1] Why do you think that no books are ever written that way?

[1] https://ux.stackexchange.com/a/48777

I actually have at least three books on my shelf right now that have monospaced fonts! Pre-TeX computer/engineering books tend to be that way.

Monospace is legible, and 12px is a great size for web content.

It's just not very convincing given that basically no site, even the one that you're on right now, uses monospaced fonts for prose. By putting every character on a grid with each getting the same weight (even punctuation!), it makes text harder to skim.

I completely disagree! Monospaced text is far easier to skim by making the scanrate of content more constant.

You keep making these assertions, but do you have any sources? Again, there's a reason why monospaced fonts are used almost nowhere. Think about it: monospaced fonts are much easier to typeset, and yet English texts almost universally use proportional fonts. Proportional fonts give each word a unique width and shape, which makes them easier to skim; monospaced text is so grid-like that you have to actually read the word to skim it (there is no "flow"). There are actual studies that have shown that monospaced fonts are worse for legibility and comprehension, and take longer to read.[1] [2]

[1] https://blog.codinghorror.com/comparing-font-legibility/

[2] http://hfs.sagepub.com/content/25/3/273.full.pdf