That's silly, there's a finite number of threats to airliners. Excluding external threats like missiles (which are themselves defended against by other agencies) you've got two basic avenues: bombs and hijackings.

Hijackings require weapons. The first hijackers used guns, so the TSA (and other similar agencies around the world but I'll use the TSA as an example) made it hard to get guns on board. So the next round used knives. So the TSA made it hard to get knives on board. With the damn near impossibility of getting knives on board plus the fact that passengers in a hijacking are now much more inclined to fight back, hijacking is pretty much fully defended against.

Bombings require bombs. The earliest bombers used suitcase bombs, which is ideal from their point of view because they can be very large. But security screening got better and suitcase bombs are too difficult nowadays. So they switched to shoe bombs. But now they're impossible. So they switched to underpants bombs. And with the latest (overly invasive) techniques that's more or less impossible too. The size of the largest bomb you can smuggle on board keeps decreasing, and the danger keeps getting smaller and smaller -- whatever you can stick up your butt it's never gonna blow a sizeable hole in the side of an airliner.

It's a finite space of threats, and they're actually doing a pretty good job at defending against them. That's why since 9/11 there have been N foiled attempts to attack airliners, and zero successful ones (I think there was a Russian exception).