Why carry an acoustic system, launchers sonobuoys etc on a maritime surveillance mission. If you stumble across a submarine you will be able to track it until it goes sinker. In peace time that should be enough. Remember we are now intelligence led! If there was a chance of stumbling across something the ac fit may be different.
We were intelligence led when I was re-tasked from Fishery Protection onto ASW with no prior warning, whilst airborne and on-task. Few maritime sorties had no sonobuoys on board the aircraft, probably not enough for a full on task period, but enough to maintain contact until the SAR Standby a/c got there.
The ability to carry out multiple roles is probably now more important, given the numbers of airframes available. You're going to need more than 4 airframes for 24/7 ASW.