The P-8 also has the advantage that it gets you where you are going faster. Its cruising speed is 440 knots (850km/h) compared with the propeller driven P-3's still impressive 328 knots or 610 km/h.
Just not with all the capability of a P-3 Orion. If anything, the P-8 has the potential to complement P-3 operations.
Interesting too: In 2008, the U.S. Navy deleted the requirement for the P-8A to be equipped with magnetic anomaly detection (MAD) equipment. This was part of an effort to reduce P-8A aircraft weight by 3,500 lb to improve aircraft range and endurance. P-8s for the Indian Navy will continue to retain MAD. Does anyone know if Australia's P-8s will have MAD?
I would also add that for a country the size of Australia, expecting 8 replacement aircraft (plus the blue-sky-marketing dream of UAVs) to do the job of 18 existing and operationally mature aircraft would need some real demonstrated proof.