Money is never wasted.
While the long explanation is some what technical and boring, the short version is this;
"Money is neither created nor destroyed, it simply moves from one hand to another".
Put another way, the National Gallery had 20 mil to spend. So they spent it. That 20 mil us now in the economy, and will travel further. The family that sold the painting might need a new roof, or a tractor, or whatever. They in turn spend the money and it flows.
An economy is just the flow of money. An economy stalls when the money stops flowing and is hoarded.
Fundamentally you want rich people to spend their money. On "what" is mostly irrelevant.
Here's another simplistic example. The US produces a surplus of wheat. USAid buys a lot of that wheat (using tax money) which is thus a round-about subsidization of wheat farmers. This is prudent because local food security, ie having farmers at all, is a good thing.
Now USAid have a pile of wheat, so they donate it to countries that can't afford it. This buys US prestige, both with those countries and their neighbors.
Now USAid stops. The govt "saves money". Farmers loose their subsidy. Long-term US citizens lose their food security.
Money itself has no value. Spending that money has value. Because only by spending it can you realize that value.