How much art is being bought only to park some money (like Chinese rich buying houses in US and Canada, keeping them empty)? They might treat art like bitcoin - as unreproducible proof of work standing in for money.
Hard to tell.
Rich people store art in freeports (along with gold bars, like in the Geneva Freeport) to secure their assets while avoiding paying taxes. And from times to times, they lend some of their art pieces to museums in countries where they want to further reduce their taxes.
According to an article in The Economist[1] in 2013: "Because of the confidentiality, the value of goods stashed in freeports is unknowable. It is thought to be in the hundreds of billions of dollars, and rising."
[1] https://www.economist.com/news/briefing/21590353-ever-more-w...