If I were hypothetically in a financial situation where I felt comfortable dropping a hundred bucks on something like that, with the knowledge that it was absolutely going to be true, sure why not. It'd be kinda fun knowing ahead of time and also potentially "leaking" the information somewhere to eventually be proven right (assuming no one else is able to pay to obtain it). Probably not REALLY worth it, but what the hell, I've probably made dumber purchases in my life.
Not really sure why anyone would be worried about the spoilers, honestly. In this day and age with the internet and how rapidly information spreads, chances are you're going to find out the roster well before you get the game, unless you go to great lengths to avoid the information (and really, is the surprise worth being forced to a) pretty much not use the internet for a few weeks/months and b) make anyone you know IRL who likes Smash not tell you anything for that period of time as well? I don't think so). At this point, there isn't even really a reason (save tradition) for unlockable characters to be a thing anymorel.