They will not lose a lot of revenue from reduced MRs because the amount of MR traffic is so tiny (and likely did not even factor into the conversation about the new rules).
Their biggest revenue risks are from frequent flyers (including business travelers) no longer paying a premium to fly on a UA flight due to reduced incentives, and switching instead to lowest cost/convenient flyer or another alliance (I predict this move will hasten shifts to the new US-AA in the DC market, for example, but that would make less sense in other places without strong alternatives).
Obviously, the consolidation in the airline industry has strengthened the logic of FF revenue requirements, as many will find themselves flying UA regardless. The goal is to extract a bit more surplus from those people.