Well, they can't say no if you are already in possession of the image.

Certainly, they are under no obligation to take the photo, deliver the photo, host the photo on a server, but once you have the photo, there is no legal mechanism from preventing you from using it. Your arguments in this thread have gone: copyright protects photos of public domain works -> well people can still sue you -> well people don't have to give you access. You have arrived at the truth: museums don't have to take photos or share them with you. They own the physical artwork and can control physical access to it. I don't think that was in dispute.