Alright I'm going to attempt to give a serious answer to the OP's question. What if Batman v Superman sucks? Let's assume that by "sucks" it means that BvS comes out, it gets less than a 60% on Rotten Tomatoes, and after it's first week it's starts dropping in the Box Office pretty hard, and in the end makes less than 1 Billion WW.
If I were a betting man in Vegas, I would say that in this scenario, Warner Bros would have serious concerns about Zack Snyder helming Justice League, and from that I think it'd be pretty likely they'd replace Snyder, but keep him on board as an advisor/producer, so it doesn't look like they told him to fuck off. Because it's asinine to think they the man be given a third chance. He was given Superman and a huge budget, and he fucked it up. Then he was given Batman, Superman, Wonder Woman, and Doomsday, and an even bigger budget, and let's say in this case he fucks that up to. If he can't handle that, with a more competent screenwriter this time, then how the fuck could he handle The Justice League?
No to mention that saying "Justice League is set for release on this date! And Zack Snyder will be directing!" isn't going to generate hype with a lot of people. The guy's name will be like a fucking tumor attached to the title. Of course there will be people who'll be excited anyway, but a good percentage won't have a lot of hope for the movie. Therefore if WB wants to achieve maximum hype, they'd boot Snyder and replace him with a bigger and more respectable name. Such as George Miller.
But I digress, BvS still hasn't come out yet so we don't know what's going to happen. If it sucks though, then I'll be betting that Zack is gonna be voted off the island.