[andrewstuart2]

I mean that to stay stable and grow in popularity, bitcoin must sustain ever-increasing energy demands so that nobody has a majority and can start double-spending. The whole goal with variable difficulty is to counteract the ebb/flow in popularity and ensure that the work takes ~10 minutes. This ensures that as the popularity grows, more energy is required since it will always take 10 minutes for a new block, regardless of how many contributors are involved.

Furthermore, the miners are here for the direct mining rewards. I think the future transaction fees approach will be significantly less attractive, so I'll be very interested to see how that change plays out once the era of rewarded mining is over.