>For some people, buying art aren't isn't too different from buying stocks
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.