The sonobuoys vs dipping sonar debate has been long-running ie
more than 30 years.
Sonobuoys are in terms of life time cost are essentially very
expensive because they are generally expendable (and in addition to
the receiver equipment needed). Once dropped (and there only 25 per
helo), it ties the helo to that general area cos one still needs to
be around to capture data. Today, there are recoverable sonobuoys
(nor is it 100% recovery) but again the process of recovery is
tedious and at the same time, inefficient for a frigate that is
constantly moving.
Whereas in a protected passage mission, a dipping sonar allows
the helo to follow the ship for the endurance of the helo ie no
need to go back to vessel to reload. Most importantly, the latest
dipping sonars like the Helras, are advanced low frequency which
have coverage way beyond what 25 traditional sonobuoys can provide
ie extremely long range. That’s why manufacturers are pitching
dipping sonar only helos. Helras has up to 60nm range scale (111
km!) with bi- and multi-static function. In other words, it is also
heard not only by the formidable which launch the helo but any
other vessel/a/c that has the receiver. The Europeans have been
crowing about how this has changed the ASW
dimension and their ASW capabilities etc.
Having both capabilities is of course preferable (and there is
no reason why it can’t be retrofit). Having said that, I’m sure
RSN/mindef/dsta did their sums b4 hand.
As for MAD, there’s still the F-50s.