The E-3's are on their knees, with recent groundings reported widely in the news.
SDSR 15 directed the E-3's to be extended to 2035, but the cost of getting them to the USAF standard will be expensive, plus the cost of improving the reliability of the current fleet. Also it is likely that some work on the radar will be required to future-proof.
The RAAF have worked hard to get the E-7 to its current capability and are undergoing a refresh of the Mission System to improve it further.
The Wedgetail has proved extremely successful on Operations and availability rates of an airliner; something most military aircraft can only dream about!
From a green aircraft perspective the P-8A is a 737-800 and the E-7 is a 737-700, but there could be significant savings on in-service costs. Indeed, many airlines run different versions of the 737, but treat them as a common platform.
MOD could make big savings by moving to a common 737 ISR platform. For example, a single IPT in DE&S? Common RTS and Safety Case? Training?
Across all the DLODs there could be opportunities to do things in a single approach rather than each platform doing it differently.