First, I'm not saying there is a right answer.
Second, I'm only trying to point out that often seemingly simple solutions have unintended consequences. In this case the further concentration of art in the largest metropolitan areas. I think one could make a reasonable argument that major metropolitan cities already have a high per capita art quotient - and that there is a public good in ensuring people in Topeka and Wichita can see great art as well.
I guess I'm really just asking the question - "Do we want to create a situation where art does not get donated to small museums in order to spite a couple of billionaires?"