The data I have shows the fuel burn per seat on a MAX 8 is lower than on a CS300, so an increase in fuel will actually help a MAX 8, let alone a -9 or -10, unless again the aircraft is being used for lower demand route and a MAX is not the right aircraft for the mission in the first place. Agree though, vs. older equipment it will provide an advantage.
I saw a guy speak back in the summer of 2014 that predicted the drop in oil when it was sitting at around $100. Saw him again this past summer and he thinks we'll hit $90-$100 again due to the shortage of investment in oil overall, but long term we'll settle in around $60 with the US (and Middle East) ability to turn on the taps and then increase advancements in battery technology potentially tampering demand. Won't claim to be an expert in any of these areas, but it seemed quite reasonable to me.