The great Gentileschi coming up for sale has prompted a debate about the ethics of museums displaying loaned pictures that are 'for sale'. Lee Rosenbaum chose a poor example to criticise in the Gentileschi because it is such an exceptional picture, of unquestioned quality and significance. It's been owned by the dealer for decades and hung in his own dining room for most of that time, so describing it as from his 'private collection' rather than stock seems fair. That said, I think she describes a real problem.
Bendor Grosvenor goes too far the other way, in my view. I'm sure he's right that museums are no pushover for loans, and he agrees that it's unseemly to sell straight after exhibition. But I'm not sure that the quality of old masters is always so self-evident. The fact that dealers go to such lengths to establish the quality of what they're selling speaks to that - the experts consulted, and the often impressively scholarly and luxurious catalogues that they produce are all part of marketing their wares, explaining and justifying their assertion of its worth. The term 'museum quality' is used by dealers to convey value, but they would say that, wouldn't they? You can't argue with the claim if it's actually been shown at a museum.
I'm glad Feigen lent the Danae, and the Met was right to take it. But I've seen other loans that are less worthy, including things claimed as being from the 'private collection of [so-and-so-who-just-happens-to-be-a-dealer]. And I've heard from the author of a monograph on a certain artist that a work loaned by a dealer was exhibited as being by said artist, although the curator who borrowed it agreed with the author that it was no such thing. That provenance (on loan to major museum with dealer's preferred attribution) adds kudos, and even if it's not reflected directly in price it may make it easier to sell.
I think it comes down to a question of balance, and the debate between Lee Rosenbaum's and Bendor Grosvenor's positions is healthy. Museums should be worried about what people will say about the loans they accept from dealers, but they shouldn't automatically turn them down or impose onerous conditions either.