I think my point about 21 or 9 aircraft may have been a clue as well.
When the program appeared to have a good chance of success a full fleet update would seem reasonable.
AFAIK there has been no sudden change in threat that would justify a reduction of more than 50% (shades of Nott). To suddenly drop from 21 to 9 is clearly either an economic case or cutting our loses as it was too high a risk.