I can't see how, exactly, people know it'll be bad from what's pretty much "hey, there's this idea we might do." I'm not saying it'll be great, not at all, but I mean, it does have that little bit of potential to be enjoyable, even if it's a sort of junk food movie. Sure, live action adaptations of toylines tend to flop, but hey, there's gotta be some exception to the rule, no?