Now think about the following potential of abuse:
An OP posts a topic with some agreeable or interesting Subject. Other users chime in and post: Yay, I like your topic... totally agree... whatever...
Later the fictional malicious OP changes Subject to something really nasty and/or offensive and above assumed update mechanics is imprinted on any poster of the topic/thread. I definitely wouldn't want this.
That's already possible: with
move topic (bottom-left), OP can
Change the topic's subject and
Change every message's subject.