If there are 10 threats out there, I might take precautions against the most likely and the most dangerous. I might also have a budget, so I would have to factor in cost.

Unless I had 10 or 15 times the $238 billion projected cost, unless I could buy everything in sight, I might want to prioritize the threats as to most likely, most dangerous.

The simplest threat might be the most likely. That would be the lowest tech threat. A "dirty" bomb, for instance. Or a nuclear "device".

The simplest "launch" vehicle might be something which didn't need obvious facilities. Like a container. Big enough, cheap, reliable, and undetectable. Comes with accurate guidance, too.

If the "device" explodes, then it would be untraceable, too.

Certainly has a lot of attractions for an attacker who doesn't have an entire nation with which to work. Or even if it does.

Or shipyards capable of modifying a cargo ship to launch a missile. Complete with delivery of warhead, missile, guidance system, mating them all, and shipping out. All undetected.

If I were building a defense system, I think I would try to defend against the most likely devastating scenario rather than the least likely, the hardest to carry off, the most likely to be detected before it happens. I might want to defend against the most real threats, first.

Granted, many things are possible. A rational plan would try to deal with the most likely, the most efficient, the sneakiest, the safest for the attacker.

Not the most expensive deterrent against the least plausible means of attack. Unless my motive was to spend money.

Arf