I don't understand what you mean here. In what way do you consider Coinbase a brokerage? Users on Coinbase can trade directly on its exchange and pay trading fees. Restricting the exchange to only brokers would mean that the middlemen ("brokers") would now pay the trading fees and pass that cost onto their customers, plus some brokerage fee (which means a higher total fee for users). The broker-intermediated trades are necessarily more costly than the direct trades.