Pierre Argh,
Aircraft are procured on requirement to generate X sorties per day in surge and Y sorties per day in sustained ops.
But your blinding glimpse of the obvious is indeed valid, an aircraft does need to be in the air to have a kinetic effect. (Unlike a B2 or F117 for example which, after it's proved itself as capable once, has an aura about it which may negate a reason for it flying. I believe they call it a deterrent.)
Good job we have Tornado as a baseline to work against, I could design an aircraft more serviceable than that old girl.
Anyway, I believe the Spifire production line is now closed and it would take as much to make the tooling again as buying F-35 so let's ride with it.