The chances of this being a "massive flop" and "sales disaster" are nil. There is absolutely no way this doesn't perform well.
And to think it could be cancelled after 6 to 8 issues? Seriously? Even if this were an outright bomb, it'd still get 12 issues, no question.
And companion books or satellite titles have been hugely successful over the years. The fact that we haven't had one in so long is only going to stoke interest in this.
Zdarsky's other Marvel books didn't sell well because, well, they were Howard the Duck and Star Lord. I expect more people will be inclined to pick up a Spidey title from him.