Mining costs, impact on long term viability of BTC Given the rapidly rising difficulty level and electricity consumption/$30 cost per transaction, is the protocol sustainable as-is, and if not what is most likely to happen in the evolution of BTC.
As Bitcoin expands there will be much greater numbers of transactions while the cost of maintaining the network will remain relatively constant, if not decrease, as power-inefficient GPUs and higher consumption ASICs are replaced by more efficient ASICs. The cost per transaction will thus drop.

Also, keep in mind that it isn't replacing nothing, it's replacing centralized accounting which has a significant cost per transaction.