Regards Tristar (or any other clapped out tanker/transport) then it doesn't matter how cheap the capability if that capability is never available! And, yes, I hear the argument about investment but not sure that any bespoke fleet of such a small size is really viable long term (and completely different to the 707/135 family for which support is still widely available).
That said I do fully take the point that the economics of FSTA really don't add up. Perhaps, when the deal was specc'd years ago, and we had a reasonable size air force, it had some potential but with a future FJ force of just 6 sqns it just seems crazy to be locked into this type of long term deal.