The Guggenheim is deaccessioning a 1958 Zao Wou-Ki painting. It's estimated at $7.7-10 million.
Because the sale proceeds will "go toward the museum's art fund," it's not the case that the work is held in the public trust, to be accessible to present and future generations.
Judge and Mrs. Samuel I. Rosenman bought the work and donated it to the Guggenheim in 1964 but, again because the sale proceeds will go towards the museum's art fund, it's not the case that potential future donors will say "Why should I give this to you? What guarantee do I have that you're not going to sell this tomorrow?" Collectors don't say that when sales proceeds go to buy more art. That's a well-established empirical fact.
So, so very ethical.