A number of things have changed since the decision was made in 2002, including:
-CVF delayed, planned In Service Date of 2012 (with F35 etc) now unlikely.
-T45 delayed, and reduced in number.
-Frigate/destroyer force (and entire fleet) cut by about a fifth.
-CEC delayed.
-RAF fighter numbers cut - again.
-The number of high value assets which need protecting has increased.
-Air launched anti ship weaponry continues to spread.
When the decision was made, it was claimed that the introduction of the T45 would reduce the risk, that surface fleet strength would be kept near to the SDR level, that there would be four RAF air defence squadrons, and the capability gap would only be six years. None of these four things are true any more.
"Mr Speaker, in the light of changing circumstances, and particularly delays to the CVF and Type 45 Destroyer projects ........... it has been decided to retain the Sea Harrier FA2 with 801 NAS until........"
It could be done with little political embarrassment, no need to actually say the decision was wrong, and much less than getting lots of people killed when they are overwhelmed by air/missile attack. Incidentally, wasn't John Reid one of the SDR team who said we did need organic air defence?