For those looking for a "sustained argument" for loosening the rules against deaccessioning, let me recommend Chapter 3 of economist Bruno Frey's Arts & Economics. The chapter is titled "For Art's Sake - Open Up the Vaults," and it begins:
"Museums keep a substantial share of their holdings hidden in storage rooms. Why is that so, and what can be done to overcome this situation? . . . Many paintings belonging to art museums … are never exhibited to the public …. While … exact data are difficult to get, it is safe to say that most museums exhibit at best half of their total holdings, and often not more than one quarter of their stock. The Prado in Madrid is a good illustration of this phenomenon. Only 1,781 out of the 19,056 objects the museum listed within its holdings in summer 1992 - i.e. not even 10 percent - were on permanent display. What is kept in storage rooms therefore constitutes a significant part of a museum's holdings."
He then says:
"To an economist … the question immediately arises as to why the stock rarely or never displayed is not sold and the receipts used for buying paintings more suitable for the existing collection or for other important purposes of the museum such as: restoring dilapidated paintings, extending the showroom capacity, increasing visiting hours or improving security and fire precautions. Such an alternative use of museum holdings would be to the benefit of all art lovers" (my emphasis).
He then moves on to a discussion of the opportunity costs of a museum's holdings: "At a (real) rate of interest of 5 percent per year for instance, a painting held by a museum and worth one million Euros means a steady flow of income of 50,000 Euros forgone each year, i.e. which could have been used in a different way. Thus, the painting under consideration could be 'transformed' into a permanent flow of 50,000 Euros, which could be spent on hiring more guards, providing more security against theft and fire, undertaking conservation and necessary repairs, organizing exhibitions, conducting art historic research or improving the working conditions for the staff and the viewing pleasure of visitors." And he adds: "Obviously, these opportunity costs are particularly acute for those paintings which are kept in storage rooms and never or rarely shown to anybody."
He then looks at some possible reasons why museums ignore these opportunity costs, one of which is the fear that the paintings sold will be lost to the art community. To which he responds (echoing a point I've made here several times): Often, "deaccessioning by one museum means that the painting is acquired by another museum, so that it is … difficult to understand why a loss should be involved. On the contrary, the museum acquiring the painting is more likely to prominently exhibit the painting bought than to abscond it to its storage rooms so that the public's exposure to art increases."
He concludes by proposing that museums be given "complete budget sovereignty, so that they can sell paintings and use the corresponding receipts freely for buying other art …, for restoration, for exhibitions etc., i.e. for any other purpose the directorate sees fit." "What matters," he says, "is that the persons responsible for public museums be given the necessary incentives and independence to employ the resources and possibilities at their disposition more freely."
I also recommend Michael O'Hare's Capitalizing Art Museum Collections: Awkward for Museums But Good for Art and for Society. From the abstract: "Capitalizing collections is practical and would lead to a variety of beneficial changes in museum practice, on the criterion of inducing 'more, better engagement with more art by more people.'"