Chapters (and page numbers) give people a cognitive clue as to where they are, when reading fixed content. Try removing the chapters and page numbers from a book, and most people will complain.
I haven't used Discourse, but in general I'm not sure pages are meaningful in forum software. The unit in a forum is the post, not the page. I read from post 1 to post N and the page boundary is always arbitrary. If forum software dropped the pagination but added post-ination, I'd take that as a very beneficial swap.