Yes, the MRA4 has flown. Last I heard, there were 3 flying, including one with full mission kit. The P-8 is a few years from flying, isn't it? MRA4 will be in service by the first flight. So you aren't comparing like with like, you're comparing the aircraft the (flying) MRA4 will replace with a civilian airliner from which an MPA is being developed. Hmm. Doesn't exactly sound like a well-founded comparison. We may as well compare the P-3 with the MRA4. At least in that case we have a real bit of hardware to compare with the aircraft in service, not a paper aeroplane. And what has the original Nimrod airframe got to do with the MRA4, which is mostly new?Im comparing it to the aircraft in service and currently flying.
Has the MRA4 flown? If not how do you know its performance compared to a 737? How do you know if the MRA4 will be reliable and cost less to maintain than a 737?
The 737 is leap years ahead of the original Nimrod airframe.
FYI, the MRA4 has a much greater range than the P-8 will achieve (more than twice as much), even better endurance, due to far superior loiter capability, better weapons load . . . . . . AFAIK the only area in which the P-8 should beat it is on speed. Systems capability is something that we can't compare so easily: the P-8 systems are still under development, & the MRA4s might be upgraded by the time the P-8 is in service.
BTW, since you insist that the modernity of the basic design is a decisive advantage for the 737 over the Nimrod, why aren't you hailing the superiority of the A320 over the 737 as a maritime patrol platform?