Google and DropBox were also laughed at because it was what a number of other companies were doing, not because what they were attempting was technologically impossible. Most outsiders had no idea that what Google was doing was technologically impossible, because, well, they heard "search engine" and thought "AltaVista".
It's the nature of innovation that you figure things out as you go along. If you have all the answers when you start, you're not innovating. And that implies that you have to start somewhere, usually where a bunch of other people are starting, and so of course things will be crowded when you're just a startup. It's all the stuff that happens in-between being an Instagram clone and going public with a wildly different product that matters.
Apple is an exception and had a clear idea of how they were going to do things differently. If you were to come up to me and say you were starting a smart phone company without a concrete idea (and the previous experience or intelligence to back it up) of how you were going to do things differently or how you were going to make an affordable phone without the scale of Apple or Motorola, I think I would laugh.