TOFO
All true enough for oceanic MPA/ASW. However, the transit times you imply (3 h both ways) indicate well over 1000 nm from take-off to station and not everyone's national strategy calls for that.
YG
Why do you call the 737 more cramped than a P-3? It's a bigger cross-section and a longer fuselage.
As for the central question of the thread: The problem with any airlifter-to-MPA/ASW transition is that it has all kinds of structure where you don't need it and not enough where you do, and that cruise efficiency is always a bit compromised in favor of STOL.