I assumed 'mystery airlifter that is neither designed, prototyped, purchased, nor tested' wouldn't be an option. The C-17 and C-5 are long out of production. And the C-130 wouldn't really expand airlift capacity in a realistic way. Nor would futuristic drones, parallel-universe space shuttles, or giant genetically-engineered mosquitos be realistic options, much as we might wish otherwise.
What about cargo monorails? We can call it the Civil Reserve Monorail Fleet.
I believe it's safe to say that in a pinch, the capacity can always be found. Any conflict that would require an overwhelmingly massive and sudden airlift operation would most likely end minutes after it began (along with most of us, I'd think)
And of course as usual, these topics completely disregard just how much equipment is transported by sea. Good thing too, I'd hate to be fighting a war while flying tanks in 2 at a time.