Macintoshes have had mnemonic keyboard shortcuts for inserting en- and em-dashes since forever: option-hyphen and option-shift-hyphen. They've been in my digital repertoire since I first switched to a Mac around 2004.

Uh, no? My writing style just happens to include a lot of em-dashes, as is very common. And it's not like I'm pasting a weird Unicode codepoint all over the place, that's just (rightly) how my Markdown gets rendered...