The point is that if you've never done it, you don't know what you need to do it. Finding a surface contact and classifying it without getting yourself counter detected (or, in wartime, shot down) is not easy, and it won't be achieved with a small boll on radar, tracking submerged subs via acoustics - and nobody, despite decades of stirling effort, has found an effective alternative - requires large numbers of sonobuoys, whilst effective on task periods require an aircraft with a long loiter time.
King Airs and the like would be perfectly decent platforms for inshore patrolling, which is - as has been pointed out above - a small fraction of what we require an MPA to do. You don't design an MPA by deciding to go small, use this kit and that kit, you design it by identifying the tasks it must perform and then designing the platform around the requirement.
Still, why would those of us with thousands of flying hours on MPA know what's needed, compared to the average Joe walking in off the street who likes playing with photoshop?
