I think you're kind of missing my point. A RPAS only ASW surveillance "solution" is at best a 60 - 80% result, but one that is likely to use current technology and is affordable.
Would I love a low-flying MPA with oodles of range, kit and endurance, dropping weapons and buoys all over the place? Damn right. The chances of getting that on the white-board - zero.
A RPAS that is in service, although yet to come into core, can relatively easily be converted to drop buoys for remote processing, and would be available to the front line in the next couple of years - yes please. Of course it's not perfect but it is flying.