So, the most important works end up going where they are seen by the most people? I think that's a reasonable consequence. Much better than hiding them away on some billionaire's private property.
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?"