There is a difference between knowing the path and walking it:
Every large market, and every good idea, will have half a dozen others who are digging around the edges of it. Many of these companies will be doing something similar to your own idea, or will provide a service that at a high level sounds similar. In some cases these companies will win if they have a superior product or an unfair advantage in distribution think Microsoft bundling browsers with their OS. In a lot of other cases, the market is still wide open even if at a high level it seems crowded or busy.
AB's Reflections 