That makes it even more confusing. Every time you click into a thread, you pick up where you left off. More casual users start on the first page or somewhere in the middle. The thread title, if updated, doesn’t match the content.
You also don’t get front page content from that kind of thread. A new thread is easier to promote to the front page.
I agree there is value in the existing threads, which is why we lock instead of deleting. Maybe I could do more to keep them on the first page instead of dropping off.