While I can understand the desire to keep old posts in place, I hope you will also understand the occasional desire of someone to delete an old post, for a legitimate reason, of course. For someone to come out and say they want ALL of their old posts removed - well, that's not likely. But people learn over time and ways of thinking sometimes change, as do who we associate with. Like right now, I think Jeff (PA) is a pretty cool guy. Will I still think that next year? Probably, but just making a point here.
Yes, all the posts together make up a thread, and each thread is its own story. Remove one or more posts and you potentially remove chapters from the story. At the same time, some folks share some slightly personal things about themselves here from time to time - and I'm one of those people. If I suddenly started getting some nut job harassing me and I think they tracked me down using info I've posted, I would want to delete or, at the very least, amend the info to remove clues and such.
So while I can understand the desire to keep posts in place, I hope you'll still allow for exceptions when necessary.
I had a post that was made not too long ago where I actually asked folks to please not quote me, as I wished to remove some of the more personal info after a few days. I was a little surprised (pleasantly), but everyone allowed that possibility and, a few days later, I deleted some info with a notation. However, the majority of the post remained as it was still a relevant part of the story.