But it’s easier to find Arab women. Or an Irish girlfriend of an Arab man. Or two old Korean people.

(All real examples)

I'm pretty sure the potential recruitment pool of young Arab men is still many times greater than the pool of Irish girlfriends of Arab men.

It's about improving the odds/reducing the exposure, not achieving some theoretical absolute perfection.

It's bad math. It doesn't matter if you have 1 arab man and 1 korean woman, or 1000 arab men and 1 korean woman. You only need 1.

If you calculate the probabilities correctly, you get very different results.

No, obviously it's harder to find two old Korean people than one old Japanese person. Everything you've listed is hundreds, thousands, or millions of times more difficult than the young-Arab-man case.

Suppose you take down a plane with a young Arab man, and then you want to take down a second plane. There is a neverending stream of similar men willing to do the job. If your strategy requires you to use elderly Korean couples, you're done after the first plane -- you'll never find a second one.

This is also absent in the analysis of "security theater." I've often felt the "theater" does in fact have a material impact on target selection. One doesn't need to actually have a methodology that results in better capture of terrorists to deter them to other targets: one just needs a methodology that has plausibility of increasing the risk of failure. The unfalsifiability of "security theater" is actually a feature not a bug: it means there's always a non-zero weight on it's potential risk impact to terrorist considering air travel as a target.

All other things being equal, the opportunity cost will shift towards targets that have less elements akin to "security theater", since it's basically 'money on the table' to de-risk the attack.

So, the real question to ask about "security theater" is not if it has a material impact on human safety with flying, but if its deterrent effect pushes risk to places we'd rather it not go or if the costs of performing it do not outweigh this deterrence benefit. Given the potentially paralyzing effect it would have on the global economy if air travel were covered in a blanket of fear of flying, it's hard to argue that "decentralizing" this risk to other targets is a bad idea.

The problem with heavily focusing on Arabs while paying less attention to other threats is that Arab Islamist terrorists aren't the only problem aviation security needs to deal with.

Focusing most of the security effort on Arabs is a good way to fight the war of 19 years ago, but it leaves the air travel system vulnerable to upstart terrorist movements that see the lack of universal security as an exploitable vulnerability.

For example, there's nothing to say that America's right wing terrorist groups won't decide to switch from shootings and vehicle ramming attacks to attacks on air travel. The TSA ought to be prepared for this, or any other, emerging threat.