The RJs were not boneyard aircraft - they were in-service KC-135s. Their airworthiness (or otherwise), sustainment and upgrade cycle is managed by the USAF and not an intellectually bereft DE&S senior leadership team.
The failure to keep the E-3D in the wider E-3 fleet block upgrade program was a major mistake. Trying to keep the E-3D ticking with in-house resources was to the considerable credit to those directly involved but overly optimistic and manpower intensive. Having a new clueless DE&S senior leadership team wondering why the (UK specific) support team was so massive when compared to other PTs, before promptly offering them up as a saving measure, was just barking mad. This left the UK not just on its own with a unique fleet, with no economies of scale, but suddenly with no UK-specific team to keep the platform going. The switch of support contractors and single track maintenance did the rest.