you shouldn't worry too much about what may or may not happen a 140 years from now since that is one and a half generation away from today! not to mention that with the way bitcoin is going, so far despite the huge decrease in the amount of block reward (from 50 down to 6.25) both hashrate and difficulty have grown to ridiculously higher values while miners are making millions of percentages more money compared to when the reward was 50!
This is solely happening because of the increase of bitcoin price right?
And what you stated is the increase from the past to the present but it still won't change the profitability of mining bitcoin and the fact that some miners have stopped mining because they can't get decent profit anymore.
Anyway, it's just a curiosity, and getting a satisfying answer will definitely stop me from asking the same question again. I won't make it until 140 years later for sure.
Difficulty will decrease if people quit mining. This will then make it more profitable for those that are still mining, or start back up. At least until the next difficulty change ...
Energy costs less now.