That's true in terms of transaction volume, because of the required interaction with the blockchain... but you don't think this might change as we wrangle with different types of scalability solutions? State Channels in ETH for example? Not a short term solution by any means of course, so maybe a centralized exchange to start is the best option, at least for now. I just dislike centralization in general, but for some uses it is currently a necessary evil for functionality ;)