> I learned to type on an IBM Selectric, I use mostly mono-spaced fonts, and two spaces after a full stop makes perfect sense.

It's still considered fine to do that when using monospaced fonts, just as using the prime symbol (') instead of an apostrophe (’) won't raise any eyebrows in that scenario.

When your goal is to make something that looks like it was typeset by someone with an awareness of typographical conventions, then understanding those can be helpful. ("Twee bloggers" didn't invent them.)