I guess I can only imagine the miners in 2140.
Well, if everything works out fine, Bitcoin will be worth quite a bit then and transaction fees are able to pay miners for their work. If mining becomes unprofitable, less people will mine, but there's no harm in that.
Chances are:
1. Bitcoin price will go up
2. Tx fee will go up
3. Number of miners will go down
4. Big bitcoin holders will mine at loss to support mining
5. A combination of the above