Profit is the main reason why people go here and I don't really believe people telling that they are here for some unbelievable reasons since even if they are not earning on signature or in the other campaigns for sure they are earning for the services or the knowledge they get here.
While majority of people is here because of the money making opportunities, there are some for which this is just an extra bonus, and for some of them money earning aspect of bitcointalk forum is completely irrelevant and they would be active nevertheless so I don't think it's good to generalize.
To give you one example: I've been active on one forum for almost 20 years and on another for approximately 10 (one is general forum and the other is IT oriented) , there are no signature campaigns or any other way to monetize your activity and yet again people are active and helpful. There are bunch of other forums with similar situation, people join forums for various reasons.
If one day on that forum there are no signature campaigns, then the forum will lose more than 90% of the current members.
I agree that forum activity would go down significantly if signature campaigns suddenly got banned, but I don't think that's in anyone's interest. But to lower chances of that happening, I think that signature campaign managers should be more strict and set bar higher when accepting the participants. Same goes for bounty managers and altcoin campaigns, as those produce huge amount of spam.
This is what theymos had to say about the subject, and why signature/bounty managers play an important role here.
I like that good forum members can make money, especially when said forum members are in poorer countries and this is a major opportunity for them. I very much do not want to destroy the sig-ad/airdrop/bounty "industry". But I am not going to tolerate people posting garbage upon garbage. If the merit system completely fails and I can't think of anything else to replace it, then my next step will probably be to completely remove all ways for forum users to make money from posting (eg. removing signatures entirely).