As soon as mining becomes "uneconomical" you will revert to the point that the only miners are 100-1000 geeks around the globe doing it for fun. That is still

As soon as mining becomes "uneconomical" you will revert to the point that the only miners are 100-1000 geeks around the globe doing it for fun. That is still enough computing power to make the network moderately secure. Entities with a vested interest in Bitcoin being unhackable by those with too much CPU power might then run non-profitable mining setups.