In fact Sig campaign may cease to exist some day due to bitcointalk advertising popularity, sad but it's the truth
Unless there's a change in the way forum ad spaces are auctioned, this probably won't happen.
There's a fixed (and limited) number of ad spaces every round, and those are auctioned off.
Now with this increased popularity for forum ad spaces you mention, given the auction format combined with the limited spots,
the price for a forum banner ad will rise, and not everyone interested will be abled to either pay that price or catch a spot, given the limit.
As a result, those who either can't pay for a forum banner advertisement, or were unlucky in the auction and can't run one for that reason,
might look into running some other way of advertising over here, including but not limited to, signature campaigns for example.
Forum ads may cost more than regular Sig campaign but at least forum ads doesn't spam or breaking the rules
Neither does a well run/managed signature campaign