Bigger aircraft cost more, specially if they are based on 50+ year old designs. Who will buy them for the next 40 years if they don't fit the requirements anyway? It just doesn't make sense.
Cost of an aircraft is not directly proportional to size. For example it is easier to create a long range variant of a large aircraft than it is a small aircraft even from a basic volumetric perspective. What adds to the expense significantly is novel architecture such as what you're proposing. I'd suggest that ultimately the military cares less about fuel efficiency than total time in the air and through life maintenance practicalities.
You're also making the assumption that the requirements stay the same for 40 years, which is highly unlikely, more likely is that new technology will come along that needs to be retroactively added. This is most easily achieved on a larger aircraft that has a larger weight growth margin and more space to fit the stuff.
There's also a strong argument to be made for sticking with a commercial airframe from the perspective of support. One of the downfalls of Nimrod was that it became an orphan fleet long after the commercial equivalent was out of service. Support for airliners rolling off the production line today will run for at least the next twenty years, if not longer in the case of common parts. None of the expensive custom widgets typically associated with aging military aircraft.
Of course it wil take E8-10 billion Euro's to develop, but a lot of technology can be transferred from existing modern aircraft. Not everything needs to be reinvented. And doing a Nimrod, P3/P7 has the export potential of a Nimrod, P3/P7.
If you don't need to re-invent the technology why not just shove it all into an off the shelf airframe and save the 8 billion? Are the capability gains you predict worth that much cash, particularly considering that most of the export sales will be taken by Boeing's cheaper alternative anyway?
Just bring in the big tankers to refuel somewhere far away. What are the total costs of having that capability globally? Plus that capability for helicopters.. we just can't afford!
That's the beauty of the A400M tanker option. It can tank anything including choppers, all aircraft are capable of a basic tanking role, the pods take a couple of hours to fit, don't prevent the aircraft carrying a full cargo load and if you want you can even buy extra tanks for the fuz to make it into a full fat tanker. Why go through all the hassle of maintaining another fleet capable of refueling when in the very rare cases it's needed we have another aircraft capable of the job.
To be perfectly honest, an MPA conversion for A400M (ideally consisting of equipment predominantly palletised in the hold with a custom rear ramp for stores, fuel pods and some external array hardpoints) has the potential to make very good sense. The aircraft is at the very start of what's going to be a long in service life. It has a large payload, long range, is quiet and we're already getting a fleet of them.
An aircraft bringing its own maintenance crew / maintenance tools, being able to refuel, function as a powerfull sensor for intelligence, able to perform a SAR and patrol with minimum adjustments saves money. Less transport, tanker, helihours, flight cycles, crews, specialized fleets, or smaller ones.
But you see that such a unique aircraft is a small specialized fleet requiring specifically trained crews capable of maintenance is hugely expensive.
MPA tanking does not make up the bulk of tanker work, we don't heli tank and we only divert ISTAR assets to MPA when they're needed. You're adding expensive capabilities to the aircraft that we barely use as it is and while it would be wonderful to have a flying swiss army knife, we can't afford one right now.
As an exercise in what's possible the design is interesting and original, but it's the hard financial realities of capability vs. cost that killed Nimrod and will kill any attempt at a bespoke replacement.