Hi :D
Since the dificulty gets adjusted dependeing on the Haspower used to mine the previous block i think of this as a tangent, but the miner will keep getting Satoshis as they confirm transaction, so there's no need to fear miners will ever be useless, they become more and more usefull each transaction :)
PS: The gold is not just "mined from underground" it's a material that can't be created in earth, i think it's only created at the heart of and explosion of a supernova and came to earth in meteorites.