A significant portion of the gold produced is bought by central banks and institutions which do not intend to resell them. This gold is effectively removed from the market. Once gold production decreases (it eventually will, when existing gold mines run out of gold), there will be an imbalance in demand-supply. There will be demand for gold from jewellers and from people who don't trust central banks (and bitcoins. )
To say that gold will become rarer since gold mines will run out of gold one day seems to me wishful thinking of the same kind that bitcoin will hit 1 million dollar mark another day. It may indeed happen, but personally I wouldn't hope for that (meaning that such a possibility wouldn't in the least affect my economic decisions)...
Why did they not run out of gold some fifty years ago already?