Well, the [End] key takes me to the bottom, but the dynamic loading makes it a pain to jump to a particular subsection of a thread. So I can’t say “hmm, I think this was on page 20” and look at it - it’s all just one blob. EDIT: The x/x counter with progress bar in the bottom-right has a dropdown that lets you select the page number.
A traditional BBS handles the insanity by locking threads after x posts and recreating them if there’s interest. I think the same is necessary in Discourse, still, since it becomes unsustainable to view posts in threads of a certain size.