A large part of this is that the move to zero-fee trades meant there's less money to maintain current operations, so by acquiring, they can, yes, "buy revenue," and save money by reducing redundancies and economies of scale. A world with zero-fee trades can only support so many brokerages.
In the past volume was king. A brokerage made money on volume. Nowadays, while volume is still important because of payment for order flow, investing the float has become a lot more important and represents a large portion of brokerage revenue.
