95% of people would leave. The rest who have a signature campaign would likely significantly decrease their time spent here. The forum would then just be populated by people who actually just wanted to talk about bitcoin without the financial reward.
I would love it if the campaigns were completely banned. But if that was true we wouldn't have many of the projects that are currently in existence practically like every single worthy alt-coin projects out there. Ethereum, Monero, etc were all started here first. If you ban campaigns, you are basically saying bye-bye to the future potential world changers. It's annoying to put up with all the crap but I think it would be really dumb to ban campaigns.
Why would alt coins cease to exist without signature campaigns here? There is at least one other board that allows campaigns but doesn't have anywhere near the traffic to make them worthwhile but they would soon get it if this one banned signature campaigns as there would be thousands of refugees that would flock there.