It has already grown since the ban of Mixers on Bitcointalk, with some of the forum's reputable members and many more having already transferred their accounts there. FYI, traffic to Bitcointalk increased when the signature campaign launched in 2011. Nobody expected Altcointalk to reach the level of Bitcointalk, but it is now on the right path to compete
Okay, let's wait and see. I don't consider altcoinstalks as a threat in any way. There is always a place for high quality posters in high paying campaigns and if someone leaves bitcointalk because they aren't good enough to participate in current signature campaigns (which pay more than campaigns listed on altcoinstalks), then they can leave bitcointalk and move on campaigns that pay $40.
Mixers are not illegal and from what I've read the altcointalk forum is not hosted on the U.S jurisdiction so it's safe to say the forum is on a safer space than Bitcointalk.
It's not only about being legal and illegal. When you let services like that to promote on your website, you'll be in trouble. When you are a small guy, no one cares but when you have a big traffic and an active audience, then they care. If altcoinstalks grow, they'll have to deny mixer signature campaigns, if they don't grow, then no one will ever care. And if they don't grow, payments will remain low.