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.

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.