At the moment, casinos are the only ones that have a long-term ongoing signature campaigns. If forum bans casinos, then there will be only one or two signature campaigns with limited number of participants and the competition will be one of the highest, ever. The payrate will probably be lower too.

I expect people to quickly turn into a drama queen. They'll start drama, start talking about how the freedom of speech is getting worse and more likely competitor forums will use this advantage to try and bring Bitcointalk traffic on their website, just like altcoinstalks did recently. Many people will leave this forum and the forum might die, it won't be as active as it is today.

By the way, there is a symbiotic relationship between this forum and the signature campaign. The forum increases their brand awareness and makes them popular and signature campaigns keep this forum active.