I'm sorry, I'm having difficulty following your point.

That's a really, really crap argument that is permissive of all kinds of cultural genocide.

Isn't it the opposite? He's arguing that work of cultural importance should NOT be in private hands. You might say, "Who gets to determine what's culturally significant? And why do we trust governments to do a better job than private collectors?" Those are fair questions, I think, but then I get lost again:

A LOT of artwork is in private collections that by no right should be.

Right, that's what he's arguing too.

I live in country that used to "legally" buy and sell people.

Ok, so... You acknowledge that just because it's legal to trade in something, doesn't mean that it's moral or ethical. So is that also true of culturally-significant artwork?

See why I'm confused as to your argument?