I know that Sam knows 100 times what I do about failing startups, and that some probably do fail for this reason. But I think this meme that "your startup probably failed just because you stopped trying hard enough" is awful.
Even Sam says IN THIS ARTICLE that "Also, very small startups can grow by sheer force of will, even with a bad product." Looking at the TechCrunch reviews, I think probably half of these companies will turn out to be pretty bad products in hindsight that could never have gone anywhere.