You are making it look like there's no emotion involved. Buying art in my opinion is mostly an emotional one, unless you're lucky and getting a Monet in a yard sale (which you admit by saying 'you can't do anything to enjoy equities hanging on your wall.'). Also, it is definitely not a logical rationale like buying stocks. Atleast not a stable one.

For half a billion dollars there are better ways to invest, and diversity. Without risking a single point of failure like here.

Also, what guarantee is there that this will appreciate? What's stopping from this painting's value from dropping to zero in a decade or two? Other investments do too, but that's where the diversification part comes in. Art definitely has it's place in the future, but a specific piece isn't worth risking half a billion dollars over.