I would agree that the role of the MR2 is far more diverse than many people acknowledge and/or realise. However, several of it's current roles could conceivably be taken by other platforms. In particular, most, if not all of it's non maritime tasks could be assumed by a combination of UAVs, E-3D or Sentinel (assuming of course that they reduce it's current mission kit weight!).
Of these, medium UAVs are probably the most attractive option due to their long endurance and small logistics footprint (although they are clearly not yet mature enough to take the ASW/ASuW tasks). The only snaggette with UAVs is the need to purchase bandwidth to get the data offboard.
I hope that we do get 18 MRA4s. However, personally, I rekon that the MRA4 will turn out to be an MR2 with the MRA4s mission kit and sensors. And I really cannot see more than 12 being purchased, of whatever type.