I'm talking about above ground investment grade silver that can be delivered to market.  You're talking about so called "deep storage" silver, which isn't even economical to extract.  Even so, you referenced (the largest below ground silver reserve in the world?) and it's only $60.8 billion.  But that's like saying trees aren't valuable because I saw a tree over there we haven't cut down and dragged to market yet.  

It requires two neutron stars colliding to make that silver, so unlike the tree, once that largest in the world silver reserve is gone, you aren't getting another one.  And mostly unlike gold, civilization actually needs that silver to function and uses it up all the time.  It also might take some astronomical time span like 20 years to extract the silver you referenced, and might cost an order of magnitude higher silver prices for it to happen.