Actually you've got it all backwards, Game Theory made their videos entirely because everyone raves about how great the story is (which is pretty damn good, might I add). There are entire YouTube channels dedicated to figuring out the details of the story, some of them with over a million of subscribers, if that tells you anything. And they started up long before Game Theory even considered doing a Five Nights at Freddy's video. Also, yeah, I'm willing to bet that a good portion of the fan base has never even played any of the games and has only ever watched videos of it, although I wouldn't say it's the majority.
In regards to the release date, though, keep in mind that the second game was suppose to come out early 2015 and instead it came out waaay before that. I have a feeling it'll be different this time since he set the date for Halloween and it'd be kinda weird to back on that, but you never know what might happen.