What do you think about this?
Higher total revenues --> more mining power dedicated to the network --> difficulty adjustment makes their profits nearly the same again. Obviously there are other factors like the development of mining technology, but on the most basic level that's what happens.
the difficulty never catch up with the value when there is a pump like this one, in fact the difficulty only increase 10-20% at best each month
and in the last month bitcoin did 2x basically, therefore in this time frame the gain was 2x more than before, the diff still didn't catch up with the value that increased, there is plenty of room for profit now
for reference https://blockchain.info/it/charts/difficulty