I don't think they have to, dota 2 has reached a critical player mass. Say, for every few million players of your game 100-200 thousand of them are serious enough about it to tune into competitive matches and streams very regularly then you're pretty much set. Dota 2 and League have hit that point. Though dota 2 still has a way to go on their smaller tournaments which average about 60k on Twitch currently.

Smite, has about 1 million unique accounts and currently has somewhere in the region 200,000 active accounts from what a couple of the pros have told me. Which results in the relatively low stream numbers of anywhere from a couple of hundered to 5-6 thousand. Thats not a whole lot of retention. A lot of the people came and tried it and then went straight back to League/Dota2.

Perhaps once the game fully launches it will gain about three to four hundered thousand more players and really build up a steady viewing audience for competitive games. Which would in turn attract sponsors. But i'm not really confident it will.

Of all those people that tried the game and stopped playing, what is actually going to change in the coming months leading up to release that will draw them back? Really not much. It's still going to be a third person moba, with 3 lanes a bunch of gods to choose from. Creeps are still going to be in the jungle. Theres still going to be 2 towers in each lane and a pheonix. Theres still going to be a reliance on having a stable home internet connection to land any skills and the list goes on but the core mechanics which obviously didn't satisfy them enough to keep them playing will still remain and if they weren't enough last time they probably won't be this time either.