Without a backlog of high fee paying transactions, Bitcoin mining becomes unstable in the long term when block subsidy dwindles to insignificance, as discussed at [1].
It turns out that many of the big miners have long term contracts with power companies to consumer power; they wouldn't save money by turning their hardware off for short periods of time. Power companies like this arrangement because it lets them predict demand better, and miners like it because they get "bulk rates" on electricity for being predictable in their consumption. The arrangement falls apart when miners turn their hardware on and off.
As a review for anyone reading, miners are compensated via the block reward - a direct grant of Bitcoin to the miner - as well as transaction fees for any transactions they decide to include in their block. (The person who mines the block gets to unilaterally decide which transactions go in the block; functionally this means they just sort in descending order of $/kb and include as many as they can)