I think there was a time when the forum never had signature campaigns and people were still actively involved.
I rely don't know what the forum will look like because I haven't ever thought of that.
On the contrary, I think signature campaigns are one of the reasons people are making positive impacts on the forum because they have to work really hard to join a campaign and most times, your post quality and other factors are put into consideration before accepting one in a signature campaign, hence, one must have strived so hard to be in a signature campaign and without signature campaigns, I think the forum might be more of a social media than a forum.
Users stay active to be able to participate in the signature campaign, and the advertiser can benefit from such advertising, so there is a benefit for all parties. Users want to be useful for this forum and try to fill it with the right content, support certain topics, so this forum is qualitatively different from the others forums, and this is what makes it interesting. Perhaps without signature campaigns, the activity on this forum would be much lower, but why think about it if everything is different here.