CPDLC just doesn't seem practicable for replacing the majority voice comms in the domestic environment. The need for quick instruction and response just isn't satisfied by CPDLC (sending certain clearances and messages via CPDLC can be quite time consuming...just to send, not to mention the added time to read and reply). It isn't terribly slow but it isn't as fast as today's VHF. I suppose some high altitude sectors could use it well but even still, there are high altitude sectors with quite a bit of congestion where the speed of voice is absolutely necessary. And I'm still a believer in the increased awareness gained from being able to hear instructions and responses from other aircraft.