Yeah, I thought so, because a few weeks ago I was browsing through some threads from at least 2011 and stumbled across a person offering to rent out his signature space, and the comments on it made it quite clear that this was not yet a thing. I don't know how I randomly came across that thread, but it was fascinating.
I'm really surprised other forums haven't caught on to this yet--it'd probably wreck any forum that did it in terms of post quality, but I don't think it's common knowledge that bitcointalk is a signature campaign hub.
Possibly hard to find the first signature threads because they would be under a different subtopic.
There is always an advantage in being the first. It would be hard to compete against this forum because it just wouldn't have the history.
It is probably both made the forum more and less successful. Less successful in the quality of posts (and more moderation work) but more successful in terms of visibility. More users = larger pool of good post as well but just harder to find.
The primedice campaign would still probably be the longest running campaign.