Modern air power and a Navy is going to seriously reduce any invasion force unless the invasion is from Canada or Mexico.
It's a shame the Air Force is crippled by a lack of F-22s and the Navy is stuck using out-of-date destroyers.
I hardly doubt the US is using single engine biplanes.
Anything that isn't an F-22 might as well be a single-engine biplane.
I still don't know where you think this "serious invasion" is going to come from without advance warning to attack the forces with a Navy and air force capable of pretty drastically crippling the mode of transportation of the "serious invasion."
I'm not making any predictions as to where it might come from.
Clearly at the moment there is no one poised to mount such an invasion. This lack of a strong enemy is the only reason we weren't conquered last year.
Countries that are unable to defend themselves only coast through history for so long. If they do not regain the ability to defend themselves, eventually someone conquers them.
F-22s are important for fighting against a ground invasion? This is right after you argued they can't do anything against a ground invasion?
Air power can not act as a substitute for ground troops. At the most it can aid ground troops.
If you wish to rely on air power to aid our ground troops, that air power needs to survive against enemy air power.
F-22s, if we had enough of them, would ensure American control of the skies (and thus the ability for American planes to aid our ground troops). The F-22s may not be the planes providing the aid to the troops, but they would be the ones ensuring American ownership over the skies.
Lack of enough F-22s will ensure enemy control of the skies (resulting in enemy planes aiding enemy troops).