There's a really easy fix to this: Load more comments on scroll-to-bottom. No button, no bandwidth used, but those who make it to the bottom of a thread don't have to do anything to see new comments. I agree with OP; the current system is suboptimal, to say the least.
NOoooo!!! Anything but the Scrollbar of Sisyphus!
The Scrollbar of Sisyphus breaks the back button (click "Back", browser remembers where you had scrolled to, can't go that far down, lands at the bottom, triggers second "page" of results) jumping to the bottom of the page, occasionally searching, and forces you to jump to the end of the page repeatedly to figure out where you were. It breaks nearly every UI expectation one has about scrolling (especially on a phone) and several that your browser has about caching.