I think you're confusing multiple things...
When you check for the "New posts" in a forum, you only see ones that changed recently. There might be one page, or two pages, or just a few.
On the other hand, if you click on the main forum link for a forum, you should see pages for all the unarchived posts for the forum. For instance, I think before I moved this post you put it in the Mac Pro forum. That forum currently has 233 pages, dating back to about 2005 (the older posts are either archived or were not differentiated into "pro desktop" posts in such a way that they could be tagged to the Mac Pro / PowerMac forum).
We do archive old posts, but they remain visible, and it doesn't happen anywhere near that quickly.
IF you do not see things this way, this is what you should do:
Go to User CP -> Edit Options. Scroll down to Default Thread Age Cut Off and adjust the setting to "Show All Threads" or "Use Forum Default" (this is how it should be set, but if you only see 5 pages of threads, you must have changed it inadvertently at some point).