It's an interesting thought but we've gone over this alot of times. The cons really outweigh the pros here.
Could you imagine how confusing a thread would be if people started deleting anything they said that caused any form of controversy or difference of opinion? All the folliwng posts about the comment would no longer make sense because you couldn't see the original.
Also, making permanent posts holds people more accountable and not as likely to make completely stupid posts.
^i think you're talking about editing the thread name, which i am not... the only thing you can edit is your reply to the thread topic.. say you made a few grammatical errors or something... maybe make it so the topic post is unchangeable... i don't know i just think this would be a good improvement to the forums.