According to a Boscombe Down chap, the Sentinel struggled to meet specification requirements. Fitting a probe would have been the straw that broke the camel's back.
He was also rather dismissive of the Army's idea of letting 'some helicopter corporal' (as he put it) fly as a Sentinel co-pilot.
Boeing E-7A is the obvious choice to replace the E-3D - anyone who thinks otherwise should be forced to study the Nimrod AEW3 saga. Airbus once offered an 'AEW&C' military derivative of the A310, to be developed in partnership with Raytheon E-Systems and ELTA, but it never even made it to the prototype stage. Planned mission duration was to have been 'over 11 hours'.