Obviously I'm assuming an equal monetary value of paintings vs land; my point is not about how many acres equal how many paintings but about the impact on other people that taking $1M of paintings vs $1M of land out of circulation has.
With respect to your second point, I think that taking each out of circulation has different effects. I would guess that most assets have a value of about what they're worth (i.e. the price mechanism works). I think Georgism is at least a valuable paradigm, and would be in favour of at least some pseudo-Georgist measures, such as only taxing land, to encourage higher-value uses rather than land+structures, and taxing all sorts of land at the same rate.
I'm not convinced; after all these collections are often already in public ownership. If there was something more beneficial to the public to be done with them, one would hope that would already be happening.
The point here is to maintain a society and some things must come first, namely material things which paintings are not. Of course, art history shows us time and time again the vital importance of material sustenance in creating and preserving works of art. If you really care about art, you have to care about the material foundations that make rich and meaningful social livelihoods possible.
>"If you really care about art, you have to care about the material foundations that make rich and meaningful social livelihoods possible."
I care about all the tiers of Maslow's hierarchy of needs, and I think that many paintings are worth a lot more than most 10k acres. The paintings also have the benefits of being able to contribute to shared experiences all over the world, and fulfilling more challenging needs.