Just because people think an idea is bad is doesn't mean it's a billion dollar startup idea. Indeed, most ideas people think are bad are actually bad - it's only the few outliers that are actually successful. Even then I think there is a ton of mythologizing around this idea that the founders were able to see something nobody else did, usually to make the founders look like some sort of diamond-in-the-rough geniuses, when in reality what they built was just a natural evolution of tech that existed at the time (successful founders usually just execute better and faster than others).

I mean, despite all stuff I've heard on HN about how a lot of big VCs passed on AirBnB, when I first heard of it it seemed like a very natural evolution from sites like Couch Surfing and VRBO that had existed for years.