You're not adding an intermediary. The broker and the exchange do different jobs. Coinbase is an exchange and a brokerage firm. Under the current arrangement, Coinbase is providing its customers with both exchange and brokerage services, so Coinbase customers are paying for both. If Coinbase was providing exchange services to brokerage firms, and these brokerage firms were providing brokerage services to retail investors, these investors would still be paying for both services (directly or indirectly), but prices would be lower due to competition among brokerage firms.