You're getting "soft", Vainamoinen.
Soon you'll be singing praises to Steam, lol.
In all seriousness though, what V (and Ragnar) said is 100% accurate. First of all, people will only be able to buy a "Season Pass", which means all 5 episodes upfront. Thus failing to deliver on what is already *bought* would be beyond disastrous (in addition to the Kickstarter "fallout"), so there's no way in hell RTG would allow that to happen. But there's very little reason to worry of something like that occuring (as likely as a meteor hitting downtown Oslo, I'd say), since -as with everything else- things are never "black and white" or "yes or no", but there's a whole lot between the two edges. Atrociously bad sales *would* negatively affect the sidestories and the extra content put into the last episodes (which would be a shame), but the main story WILL be delivered as planned.
What he said.
But yeah, atrociously bad sales would have a detrimental effect on books three and onwards — not to the point of breaking the story, but we wouldn't be able to put as much time and love into those books as we will with the first two (which are 95% and 70% done). So we're really, really hoping for decent+ sales. I mean, we're hoping for GREAT sales, of course, but I'm a Negative Nancy* and I like to worry and prepare for the worst. I'm a very, very cautious optimist.
In other words: go forth and spread the word! Tell people about the game! Be our crusaders! We'll give you badges and fancy names!
* I'm not sure why Nancy gets all the shit. One time, one time, she had a bad day and she was all doom & gloom, and from that point on they called her Negative Nancy. It's bloody unfair, is what it is.