Dave
How about more than 1x RPAS then? There is such a thing called Multi Aircraft Control (MAC) where you can control up to 3 RPAS from one cabin - 2 dropping sonobuoys and 1 with weapons to finish the job (you could even have it engines running on the runway).
Three Pred B sized RPAS and all the Gnd Ctl Stns with associated support would still cost about 1/5th of an MRA4.
By the way, the processing would be done offboard the aircraft through digitally multiplexed datalinks - listening to multiples of them from the Gnd Ctl Stn would not be too problematic.
I would wager that we'll see ASW RPAS capabilities before 2020 from some nations.
LJ
PS. Enjoying this debate as it stimulates new ideas.