"In late 2003 I saw a talk on the RNR Air Branch. Ex RN Aircrew, Air Engineers and others are attached to various units and establishments, operating every single aircraft type in RN service. These included former Sea Harrier drivers who flex airliners most of the time, then for a few weeks of the year back on the Sea Harrier. Similar story with the Engineers etc. Now if you can fly a Boeing 757 all year and then a Sea Harrier for a few weeks to keep your hand in, why not fly a GR9 all year and then the Sea Harrier briefly? And the same for the maintainers. And if (when?) a crisis does come along - take a few in the CVS along with the GR9s? Or instead of some GR9s (depending on the crisis)?"
Because manpower is only one element in the equation - and probably the easiest to solve.
To be operationally useful the SHar needs a major upgrade. (And the engine?).
To remain in service the SHar needs an ongoing spend on costly spares, maintenance infrastructure, ground support equipment, 'Major' tracks, etc. It is by eliminating all of the type specific logistics tail that you leverage the savings to be accrued from retiring the aircraft. Maintaining eight, four or even one SHar means that you don't get the required savings.
The CVS aren't big enough to deploy a useful mix of SHar AND GR, it's an either/or choice, and GR is more useful more often. CVS with sufficient SHar to mount a sustained CAP is the ultimate self licking lollipop.
Don't get me wrong, if it were up to me, I'd upgrade and keep SHar just in case, given unlimited funding. But when we're losing PR9 and Jaguar (two far more useful assets than SHar) arguing for retention of the Sea Jet is asinine.