This is the point of transaction fees in bitcoin. Fees get 'attached' to the block that is mined and go the miner. So even when all bitcoins have been distribut

This is the point of transaction fees in bitcoin. Fees get 'attached' to the block that is mined and go the miner. So even when all bitcoins have been distributed there will remain a profit motive for mining. The difficulty in mining is also automatically algorithmically adjusted, so if the industrial scale miners move onto other things - suddenly it might become profitable to go back to GPU mining. And the fewer people mining, the more profitable it would become.