In fact, you are right that if there is no signature campaign on the forum, the current traffic of the forum will decrease a large percentages from this traffic. Because no matter how much people or all of us say money is not everything, but money is everything in the world. If you see, all the activities of the world are for this money. It is true that those who are legit members will remain active but their activity will also be reduced to a large extent.
And if you want to see the real proof of this, then check the traffic of altcointalk forum a few days ago and now check the traffic after the mixer's campaign has been transferred there, you will understand by yourself.
However, from my personal view, it depends on the campaign manager whether the signature campaign will generate spam posts on the forum or reduce the spam posts on the forum. If the campaign manager strictly fulfills his duties and is anti-spammer then the signature campaign will reduce the spam on the forum. Because then for staying on the campaign the campaign participants will drop spam posts in the campaign.