zonoma has hit the nail on the head... if every aircraft was equipped and able to use CPDLC reliably, r/t congestion would be significantly reduced. It's a source of frustration to London controllers that CPDLC hasn't been developed into something which could really benefit controllers and pilots alike.
Why is there this mad compulsion in ATC to move away from direct voice communication to typing messages on keyboards?
Typing... how quaint. Most CPDLC messages can (or at least should be able to) be completed with a click of a mouse. Imagine the savings in r/t loading when we frequently conduct conversations along these lines:
Controller: ABC123, contact London on 123.450
Voice from Ether: *Blocked*
Controller: ABC123, contact London on 123.450
Pilot: Roger, London on 132.450
Controller: Negative, ABC123 contact London on 1...2...3...decimal...4...5....0
Pilot: Ah, 123.450 thank you
Controller: Confirm that was the ABC123?!
Voice from Ether: London, good morning.....
Repeat ad nauseam...
LTP