It is not unheard at all of for a neglected specimen to deteriorate in such storage. It is not prevented by any surface applications of anything.
Unsalvageable specimens are, well, just that.
Yes, any vulnerable specimen left unchecked and untended in poor environmental conditions can be damaged or destroyed by the oxidation. The plaster jackets won't slow this down much if at all. I'm not going to defend or reject either approach. The inner and outer faces of the museum are very different enterprises. That does not mean that collections are being squirreled away by sociopathic little researchers.