PS: as to national treasuries purchasing silver, the way things work these days, if they do buy it, it'll be to mint into coins to sell. Governments aren't really big on precious metals (for "backing currencies") ever since currencies learned to float. It's far more "flexible" to be able to print money on demand. Modern currency is a debt instrument, rather than a token representing physical value (i.e., metal)
I would not worry about the availability of silver.