This is very far future (2140), but I can try to predict.

The miners will be still needed but they will earn only transaction fees so most of them will move up to mining another PoW cryptocurrencies. If the miner is powerful enough for another crypto, then yes, they will mine it. If no cryptocurrencies will be that you can mine with this miner or miners will be too weak, then they will sell miners. Also the difficulty changes every 2016 blocks so remaining miners MAYBE can stay profitable. We can't predict that.