The original concern was not for the user, but was for you the website owner.
Changes made to the site will likely result in less page views. This leads to less ad impressions. Which leads to less ad clicks. Which leads to lower revenue for the website.
Click based ad revenue is directly tied to the number of page visits.
But as far as the user experience is concerned, I'll never read a post for airline X if it is hidden away in a subforum, I just don't care enough to make the effort to go into the subforum for an airline I have no interest in. But if it is there right in the open without having to do a lot of navigating, and it looks interesting, I'll click it to read it. And who knows, maybe there will be an ad there that looks interesting to me enough to click.
Question- The new posts tab- Will you see replies to current posts as a new post?