This is not entirely true, yes gold and silver are uncommon due to being heavy elements created only when a star goes supernova but the universe is huge, once humans are able to get out of earth and begin to populate other planets natural resources will stop being a limitation, this is why we will need something like bitcoin because no matter what you do there is a limit in the amount of coins.
True, and in fact you don't even need to go that far. Science has already succeeded in using radioactive reactions to create gold (so alchemy has succeeded!) So the currently very high cost of such lab work will become an upper bound of gold prices.
But all of these ventures are today expensive (or speculative,) so gold holders need not worry at this point.
The important thing is that the special commodity of money arises naturally from economic activities, since barter is so inconvenient. If and when all monies have lost trust in one way or another, the economy will use a commodity, if it has to (tobacco, sea shells, etc. have all been used in the past.) The state will of course always try to issue its currency, but due to the inherently perverse incentives for the issuers of state currency, there will always be a role for non-state money.