Even assuming they can find buyers for enough of the collection to pay the taxes, that's an enormous, instantaneous pile of stress on the children of a parent who just died and is probably missed tremendously.

Hey kids, I hear your parent died? Great! You owe us 500 million dollars, get to work.

I think estate taxes are defensible, but the way this particular estate was handled doesn't seem defensible. Out of all the things someone could do with their vast fortunes, buying art and then loaning it out to museums and collections doesn't seem like the worst thing in the world.

In particular, the article points out that black market valuations may have even been considered when valuing the art in the estate: That means the government is basically encouraging these works of art to be sold on the black market (likely to private collectors who will keep them in private), where if they were at least sold on the open market - or in this case, held by the original owners - they would be more likely to be put on loan to museums so the public could benefit from them.

Alternately, in a case like this where it's illegal to sell the item being taxed, the government should offer the option of simply handing it over to them in order to erase the tax liability, or giving it to a charity, in order to conclusively demonstrate that no profit is being made from the work of art (black market or otherwise).