I don't fear this will be a big issue by then, when Bitcoin becomes way more popular energy efficiency will become a more important factor and chips will have to become more energy efficient if the miners want to make a profit.
THats incorrect. It doesnt matter how energy efficient you make them, once these hardware prices have come down to something near marginal cost, electricity cost will be the overriding factor for miners, and the only real limit on network growth. More efficient chips would just result lower mining cost per TH, which will results in a proportionally higher network speed, and thus rendering the higher efficiency pointless. Have a look here:
https://bitcointalksearch.org/topic/finally-a-correct-endgame-difficulty-calculator-295270
For a given BTC price and electricity cost, you can pretty much calculate how many megawatt bitcoin will consume. The only way to prevent that would be if hardware became more expensive somehow, so that the hardware investment would be a bigger brake. Not very likely with asics.
If enough people take their miners offline because they don't make a profit difficulty will drop and then it will be (briefly) profitable to mine again. It then helps if you have the best energy efficiency. I guess when the limit is reached we will see cycles between high difficulty and low difficulty ad infinitum, until quantum computers come online and the arms race starts again.
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