Mining always needs to happen. Once it stops producing coins, the cost burden just moves over to the people using the currency. The mining cost only decreases if the proof-of-work difficulty decreases.
If mining becomes less profitable, fewer people will mine and the proof-of-work will tune to be less difficult, but the cost never disappears and there's still a huge amount of energy wasted in the selection process for adding blocks. If too few people mine the currency will be totally destabilized.
If mining becomes less profitable, fewer people will mine and the proof-of-work will tune to be less difficult, but the cost never disappears and there's still a huge amount of energy wasted in the selection process for adding blocks. If too few people mine the currency will be totally destabilized.