The real question is: Is the P-8 good enough for role.
Boeing and the USN have taken a low risk route, there is a significant amount of P-3 stuff in it and the mission system is a variant of that fitted to MRA4. In effect British tax payer has helped out the USN plenty with that. Any legacy systems can be replaced at a later date when funding permits. As for the airframe and configuration might not be as ideal as the P-3 or Nimrod in role but has some significant advantages when it comes to commonality with the civil world. The Boeing 737 is one of the commonest airline types on the planet, any country operating the type will be able to shop around for spares heck they could even buy retiring high cycle B737 used by the airlines (Ryanair recently retired its first B737-800 series) sit them at an airfield and use them as parts hulks! Nimrod and to a degree the P-3 are single source when it comes to parts. In respect of Nimrod MRA4 with less then ten to be completed BAE Systems would pretty much of had the RAF over a barrel when it came to spares.
As this is always about Nimrod in the end looking back we have to accept that the process that led to its selection and the following mess was flawed.
BAE decided it was a contract that had to be won, they dangled an aircraft under the nose of the RAF and treasury that sounded very attractive. An ideal airframe and engine configuration in the RAF's eyes and the apparent savings of recycling components for the treasury. Any consideration about development issues of recycling elderly airframes or the through life costs of adopting a small number of a unique type were ignored. Of course it should be remembered that the preferred solution at first was the P-7 which in itself was a warmed over P-3. When that was canned then we had the contest that led to Nimrods selection. I do wonder about the sincerity of the RAFs desire to have the P-7 considering that the eventual contest had two different P-3 based solutions one involving rebuilds by Lorel and the other involving new build from Lockheed Martin! Presumably Lockheed Martin didn't reinvent the wheel and used a significant amount of the working from the de-funked P-7 program.
I feel BAE should of looked at alternatives to the Nimrod Airframe preferably based on a civil airliner type. My personal favourite is a solution based on the Boeing 757. Boeing had already done a significant amount of work to develop that aircraft as an ASW/MPA type as a rival to the P-7. It was spacious, long ranged, has British engines and operated by many airlines including British Airways offering clear maintenance synergies. I have heard the main barrier to operating an aircraft with podded engines for the RAF was ditching characteristics. The counter to this is, what is the survivability of the crew with Nimrod (in the North Atlantic in a storm probably low) when ditching and is the B757 good enough for the role. There is another thing as well, the B757 would of made a perfect, cheap replacement for the VC10. Just imagine the maintenance and cost synergies if that had happened...not to mention troops flying out to A-stan now in a modern reliable airline type! Plus you have the added attraction of a possible USN purchase to replace the P-3 with British companies being major sub contractors.
Considering the P8 debate now I wonder if more pragmatic decisions had been made all those years ago we wouldn't be in a better place now. Imagine a B757 based ASW/MPA operating today as well as a transport variant...