At the moment, they don't really have any competition or at least any that markets their services very well.
Potentially another manipulated market such as the argument about Mt.Gox bubble upthread. We could be witnessing a cashing out from recent altcoin bubbles while buying from himself to create the illusion of a rising price while doing so as they ostensibly did for the ETH bubble, e.g. Vitalik using Coinbase to cash out from ETH -> BTC -> USD.
Free markets = decentralized markets.
When you have whales and most volume going through 1 or 2 exchanges, you don't have decentralized markets.