HN has collapsable threads and uses tables for layout, which aren't exactly efficient markdown, so makes the opposite point you're trying to make.

Namely, even with bloody awful markdown, HN still delivers 1000 comments in 1/4 of the bytes.

Another interesting idea, as almost everyone doesn't actually comment, wouldn't it make more sense to load the editor javascript dynamically? As far as I know, the silent majority read comments but never reply. In fact, I vaguely remember Jeff even having a blog post that most people never participate, they just consume (could be wrong!).

Also, I haven't kept up with it, but mobile browsers used to have fairly aggressive cache invalidation to save space on the phone, so serving big chunks of JavaScript in the hope it might get reused one day wasn't actually a great idea.