Signature campaigns on this forum will last till we have as active users as LoyceV, Philipma, PowerGlove, Skeptical Chymist and etc... Their number is huge and I can't include all of them. It's very noticeable when such active members leave this forum,
Important members you have listed here, no doubt; however, the existence of signature campaigns in bitcointalk does not depend on any user, what matters is the all round traffic in the forum. Funny thing is they complement one another, campaigns and traffic; if there is traffic, campaigns will come, and campaigns help to generate traffic in the forum.
Also, the activity of campaign managers play a significant role. As long as signature campaign managers actively contact casinos and negotiate with them to run a signature campaign on this forum, they'll be here. This forum still has good traffic and is beneficial for SEO too.
I don't think this would ever be a problem, there are enough campaign managers in the forum, and there are a lot still waiting to make their mark as campaign managers. So they will surely contact casinos, because it benefits them, the casinos and the forum.