Yes, it's possible to destroy or transmute gold, but as you already stated, it's prohibitively expensive thus with respect to scarcity it might as well be impossible. Yes, that might change in the future but there is no reason to believe that it will.
No. Your belief that the bitcoin community will never choose to alter the supply of bitcoins is not the same thing as the properties of physics which constrain the practical transmutation of gold.
Any change to bitcoin's supply algorithm requires a hard fork of the blockchain. People have done it before but those forks are considered worthless by the community.
You're right that it is a community belief. If enough people were convinced that supply should be increased, they could fork the blockchain and move to the new chain. The change in supply would alter the price appropriately.
But that's no different to the community belief that it has any value to begin with. If everybody was convinced that gold was worthless (outside of its practical industrial uses), it would cease to be a store of value too.
