If they obtained it legally, it is their stuff. If not legally, then it should go to the rightful owner. I’m free market even if I don’t like the fact nobody will see these paintings and other works of art for decades. If I own a painting, I should be able to do with it what I will.
This doesn’t stop with artwork...I know of a fellow in Michigan’s UP that owns a railroad, and he has a load of RARE locomotives and passenger cars squirreled away in a former shop building, forbidden to the public. Though in this case he has some legitimate reasons for his attitude (past theft), it’s the same situation...his stuff, his rules.