I'm asking if there is a rational non-meta reason to believe they do "stop at the neck"

Not to my knowledge.

Are personal traits as important?

That is a more controversial subject. They are clearly less important than intelligence, but things like time preference (what kind of trade-offs do you make between a smaller reward now and a bigger reward later) or, say, propensity for violence got to be at least somewhat important.

I'm asking if there is a rational non-meta reason to believe they do "stop at the neck" even if we throw away all the IQ/nations data.

Of course there are. The standard argument is that the history of human evolution suggests that increased intelligence and favorable personality traits were strongly selected for, and traits which are strongly selected tend to reach fixation rather quickly.