If it does than, then it can get out-competed by Rainforest, which is a clone of Amazon but with lower fees. Then if Sahara comes along and copies Rainforest but uses the blockchain for payments, they won't be able to compete because they will have to pay for blockchain network transaction fees and potential blockchain network backlog (unreliable payments compared to say funding a Google Wallet or some such)

That depends on whether the market leader benefits from network effects. All other things being equal, the market leader in a naturally monopolistic industry, like say social networks, has a much more compelling product than competitors. That allows the market leader to extract monopoly rent.