This is a somewhat theoretical discussion but I am of the opinion that especially in this case it is demand that attracts supply and not the other way around.
If the sig camps go away, only the same old people who attracted the sig camps in the beginning might post again.
If signature campaigns go away this forum will look like a cemetery. 15 years ago people hardly used social media compared to now, and forum traffic was much higher. To think that good campaigns will come back without taking this into account is to leave out an essential part of the equation.
What advertisers need is more readers, more potential customers for their services.
I agree with the latter but not with the former. Nowadays a new cryptocurrency exchange, to give an example, is going to get a lot more customers if it advertises on a Youtuber with millions of followers than by advertising on the forum.
Raising the quality of forum posts will not change that.