My guess is that the trend will be to attempt to balance fares more going forward. I expect two things...
1) Further capacity cuts (the people who need to fly will continue to fly), eliminating the seats taken up by the MR people, and increasing the average fares
2) A likely flattening of the fares somewhat to attract more mid budget people. There will always be the full fare purchasers, but I would expect that if right now the discounted fares range from ~$200-$800 on a given flight, that it might be more like ~$300 or $350 up to $800, but with the vast majority of the seats towards the lower end. In other words, maximize filled seats, but don't hemorrhage money in the process.
I am also convinced United will not be alone in this effort.
Edit: But to directly address the question, not much. Furthermore, what they do lose is revenue that they probably really don't want in the first place.