Just a quick add on as well: Mining doesn't become too expensive.

The blockchain is governed by the difficulty, which can be thought of a function of the amount of hashrate there is or the profit that is gained from mining. Those who find that it is not profitable to mine will drop out and result in a drop in difficulty, which makes mining relatively easier. The reverse happens if it becomes too easy.

However, it is a real possibility that Bitcoin becomes insecure if there isn't sufficient reward. This would be a problem, possibly in the far future but I'm sure that the increased throughput and adoption in the future will still allow mining to be compensated well enough.