As I, and Mike G have said, that claim has been made many times. Conquerors will always need 'boots on the ground' or equivalent. And I hope we never have a 'Destroyer' with the power of one of today's major militaries.
Short of nukes, this doesn't work. Look at wars within the last 50 years for examples.
It wasn't effective at much beyond destroying infrastructure. Digging into the details, it was used to push a political decision. It didn't conquer the country.When I was a kid, we watched in awe the NATO campaign against Serbia. A whole nation was put on its knees with nothing but air strikes.
Sure, if we speak money, it was costly. A lot more than if we had simply massively given weapons and ammunitions to Albanians and Croatians and say "-Go !! Now is your chance !!". But NATO took very few casualties.
Not in modern armies! Training alone can run a hundred k or more...then start adding equipment....Now, if your aim is to destroy your enemy at the lowest financial cost possible... well, ground troops ARE cheap. Especially infantry.
Or there are simply other equivalent or greater powers who won't condone wholesale slaughter.And if your aim is to control a territory without butchering the locals into submission, then yes, you need ground forces to hold and police said territory.
But this means that your nation is either too poor to afford an air force or is a respectable democracy that can't afford to slaughter civilians. Or both of this.
If it matters, I'm a USAF veteran. Even so, our generals (mostly) learned from Vietnam (and more recent wars) that air power alone isn't enough.