Wise words Biggus.
As someone with some knowledge of both ASW/ASuW and RPAs, I would say that UAVs are outstanding for the ASuW task (and are already being used, with more countries interested) but it will be a looooooong time before they do ASW.
So which do you want your MPA to do? If the requirement is a lot of ASuW with the occasional bit of ASW that the helos can't handle then there may be an argument for a mixed fleet which could work out cheaper. Or not.
Bottom line - we need something and I can't see it being a P-8 / MQ-4C mix!! Refurb'd P-3s anyone?