Paul, you're actually going a little too far. No court case has ever shown that a "museum's assertions of license are invalid," and these are case-by-case issue

Paul, you're actually going a little too far. No court case has ever shown that a "museum's assertions of license are invalid," and these are case-by-case issues. So Bridgeman may or may not apply in any given case. It could be cited as precedent for sure. But e.g. I'm pretty sure it specifically limited itself to 2D representations of 2D works. When, in the past, good-hearted souls have scraped museum websites to construct larger images (e.g., from the old-fashioned zooming tools that only showed part of the image at a time), the museums in question stopped short of pursuing it in court for fear of just what you say. So it has not been tested (even though most thoughtful people agree with you about what would likely happen). The Met has had (most of; maybe almost all) these images online since 2011. The licensing was unclear then--there was a hard to read, existing T&C that was more or less NC-BY-SA. Or maybe it was "no derivatives," I'm not sure. But getting the images out (hard but straightforward) without dealing with changing the license (hard and complicated) served basically 100% of the people without putting the Met at any risk. This is now the third time in six years the Met has gotten a lot of news out of the "release" of these images. One could argue that this is not a big step, but if, in going CC0 it means they can release more images, that's a good thing. I haven't seen any report that the number of images has changed, actually. However, it does sound like they're not claiming copyright on 2D representations of 3D works in the public domain, which they certainly could do.