> This is especially bad if they turn out to be seriously vulnerable to any missile system developed that isn't ruinously expensive per shot or a closely held secret used only by somebody's elite guard
Or if, say, the very large and expensive amount of fuel used b supersonic aircraft can be cut off by the opposing force bombing the oil lines from their own country that we relied on to get cheap fuel. It's a bit of a conundrum when the country you're invading is a major source of your fuel. Or if what you need to "win" the conflict is troops and engineers and nurses on the ground to re-establish water, food, and medical supplies after a decade of civil strife.
$500,000 missiles that can hit another supersonic craft at speed is a complete waste of resources in most modern conflicts. The more sophisticated US craft, and their pilots, very effectively cleared the air and the ground of Iraqi and Afghanistani armor and military vehicles in the last few wars. But I'm afraid the lessons of Vietnam and Korea were ignored. Successful air campaigns lead to wars of occupation, and both countries have _centuries_ of experience of outlasting foreign invaders.