It's not Dixon's fault..
There is currently no must-have need to upgrade. Got an iPhone 7, Galaxy 6? You won't be any better off with an X or 9+. The battery life is no better the phones are no thinner, The camera is incrementally better but the old one was good enough as a phone and will never be as good as a proper camera.
Couple that with two year contracts and good SIM-only deals and why upgrade? The new generation of folding phones, particularly the new Moto Razr, might change this, but just as DixonsCarphone can't take the blame for the slump it can't take the credit for the recovery.