People frequently think that their comment has been deleted because it doesn't show up on the first page. We introduced it as a performance workaround, so if performance recovers, I'd rather stop. If rendering the whole page causes other trouble, we can reconsider the problem from scratch.
Why would pagination be a good idea? From my perspective, there's no downside (HN is exclusively text and fetching+rendering the entire first page of comments for this submission takes the same amount of time on my computer as rendering a no-comment submission) and several upsides (no need to open several tabs to see all comments, no need to make several more round trips and wait several more seconds per page of comments, no "attention cliff" where comments on the n+1th page are significantly less noticeable than those at the bottom of the nth page, allow pagination to be handled by user agent/browser instead of being enforced by server).
The downside is having to serve larger pages which consist primarily of content which will not be read. This site is running on some resource-limited hardware as I understand it, so limiting the maximum potential size of each page served means more pages can be served more quickly, especially if you just cache the first page of a thread (which is all most people will engage with) rather than the entire thread.
You're confusing cause and effect - most people only interact with the first page of comments because they're on the first page, and they don't want to click through. If you disable pagination, then suddenly far more people will read those comments that would be on the second page.
Additionally, request count matters more than data transferred. It's much easier to serve 1MB to each of 100 users than 1KB to each of 100K users. n people are already going to view the comment thread for a submission - a several-dozen-KB increase in the amount of data that you send each of them (assuming you're serving them statically) results in anywhere from "little" to "imperceptibly" additional CPU load.
Why? Pagination seems like a good idea for really long threads.