Where monospace was pragmatic for the typewriter era because of the limitations of the technology, proportional is the pragmatic _and default_ styling of the web.
Choosing monospace today for prose is an opinionated, and in my opinion suboptimal, choice.
Sure, there are better typographical choices than monospace, but even bothering with that decision is like optimising a 2*x to an x<<1 in code that's hitting disk anyway.
(on the flip side: it doesn't matter how easy it is to read a sequence of words, or how beautiful they may be, if they don't communicate anything; the code analogy is that it doesn't matter how much faster a buggy change runs)
Does that make sense?