Of course traffic would decrease hugely and people would flock to the next best thing but it wouldn't kill the forum. The forum will still be here and it will quickly turn into a utopia compared to what it is now. There's plenty of posters here who don't have or ever intend to join a signature campaign and in fact many of the best posters don't have a paid signature at all (DannyHamilton etc). I'm sure theymos has taken into consideration the huge effect banning signatures would have on the forum but he has also stated signatures will likely be the next to go in the merit system fails so for those thinking they'll never go I wouldn't get too cocky.
Traffic rise mainly based on spammers have two-side effects.
It can help to attract more newcomers to the forum based on results they get from searching using google data base. These newcomers might contribute to increase passive income to the forum, but they can also get money out of the forum from signature campaigns (which dominate traditional advertisement banners).
In contrast, spammers can form negative effects by their shitty, spamming threads which might abundant in the forum. Those shit they created will contribute to make ugly appearance of the forum; and can hamper potential newcomers to join the forum due to they won't have good impression at the first glance around several topics.
To conclude, signature campaigns should be allowed but should be stricter managed (on both signature campaign managers and supporters).
More of new comers only want to have income specially crypto currency is very popular but investors are more rare. Although both of them could really contribute spamming the forum, but merit system can really eliminates accounts that cant help the forum because members could not maintain their accounts because of quality post that requires understanding which could lead every new member to be an investor.