"It all (already) exists in some way or another"... which makes it all about changes in opportunity, which mostly are uncertain/unclear over something like 10 years (AirBnB was founded in ~2008) and superior/excellent execution. So its all about execution, adjusting trajectories and being lucky, that the wind of change blows your way.

Eg. I absolutely don't want to downplay Mark Zuckerberg's contribution/genius, but Facebook comes to my mind, which certainly wasn't the first (attempt) at a social network. And: What are they today?! - Kind of the "web" (on phone/mobile) in India for example. With all the social integration and native (applications) first on (Android) phones, they became a better version of the WWW in less developed regions of the world.