Nothing is totally impossible to happen. If it can be conceived, it can happen. With the recent exit of Bestchange, it can be seen that more than 97% of campaigns here are casinos. So, if casino campaigns are banned, that's actually the end of signature campaign in the forum.
But BestChange was not forced out of the forum, they decided to end their signature campaign for advertising reasons and they still have an ANN thread here. People tend to forget that this kind of thing happens frequently with all sorts of services when they run a signature campaign, whether it is not working out for them or whether it is successful.
If I was asked to vote on which I think was most likely to happen between casinos ending the advertisement and promotion in Bitcointalk for personal reasons, and then Theymos banning Casinos, I would go with casinos just ending their promotion here just like bestchange did because with the AML rule guiding casinos, It will be difficult for them to have issues regarding money laundering that will make the government go after them.
Maybe Bestchange ended their campaign here because of financial reason like to cut cost, Casino's can choose to do that as well, also if they don't see the campaign as productive to their marketing.