Just because the US Army might go to a new Apache in due course does not mean that Boeing will stop supporting the D for other customers.
(Take a peak at the wide variety of models and variants that Sikorsky supports in the S-70 / Blackhawk / Seahawk series. )
This story smells of fish.
The issue here isn't that the US will withdraw it support for the AH-64D, but that it will not support the Block 1-standard aircraft that the UK operates. The US will operate and support a fleet of AH-64D Block II and AH-64E Block III, leaving the UK out in the cold with the WAH-64D Block I.
The Dutch saw this coming some years back and decided to upgrade their Block I Apaches to Block II for this reason. The UK too has known for a long time that this was coming, but has continually procrastinated on making any decision.