The default is for threads to open with the initial post and then subsequent posts, oldest to newest, in chronological order, as was the case with the Forum before the update. The initial post that starts a thread always appears at the top of the first page, no matter how the following posts are displayed (oldest to newest/newest to oldest).
The default is accommodating for new members. Switching this would mean erecting a barrier to people joining the Forum in the future. Current members are experienced Forum users, and are better equipped to navigate the technological hurdles presented them than the typical newbie. At present, it will remain as is.