It doesn't matter how efficient hashing is. Miners will use their earnings to buy as much equipment and electricity as they are paid for. The block reward is fixed, that's the problem. It doesn't have anything to do with hashing efficiency because the network will just raise the hashrate.
Somebody figures out how to make more efficient hashing machine -> people will buy more them -> due to increased hashing power, Bitcoin network raises the hashrate -> hashing is again as expensive as it was before more efficient machines were invented.