Interesting to observe from that really big successes are better executed versions of ideas/products that exist on the market in a big way already (e.g. airbnb, facebook, dropbox, google) and the reason they seem like a bad idea (to investors) is not because they offer something nobody wants, but because they already exist in a too-big-to-challenge of a way? And that they might also seem bad because their starting point might seem like a niche (facebook).