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The other factor, which I think is probably of some significance, is a desire to remind the public (or let them know) that these items exist and are not new. In the modern era there is always a section of the public which will query any new jewels or other signs of conspicuous consumption, which can cause a political headache. QEII has reached a state of public esteem where seeing her in "new" jewels did not cause a public outcry about the royals' wealth or their cost to the "public". I suspect that her successor will not be so fortunate, and so displaying these items now may take some of the pressure off her successors in time to come.
I struggle to believe this to be honest...such criticism could simply be resolved by saying the piece has been owned since 1942. I also think based on how utterly useless the House of Windsor is at press management such an approach would show a degree of planning and cunning they are simply incapable of. I think you give them too much credit!
Also criticism regarding new jewels worn by the royals seems focused on the fact that they almost always originate from a Middle Eastern dictator.