I would imagine that at some point, maybe in 100 years, mining may not reward you more than the cost of electricity to mine. Miners won't bother continue--how will bitcoins continue to be hashed? thanks apeace!
The invisible hand of the market. In 100 years, if bitcoin transactions are still of value, then miners and people making transactions will come to an agreed upon transaction fee that will be suitable for both parties. And If miners drop out, transaction fees will go up.
Transaction fees will keep increasing (because of demand) and we would have moved to renewable electricity by then (free electricity), therefore miners continuing mining.