If you watch UA flights board, there are often a number of people in Group 6 that brought a carry-on bag and end up paying $65 at the gate to gate check their carry-on.
I think I've seen that once in my entire life. It's a massive failure if it does happen -- it's why BE passengers have to check in at the counter, so the counter agent can remind them about the policy and check their bag for them without forcing the higher, at-the-gate collection.
I also wonder how this will affect crediting to B6 (and whether there may be a merger offer), since I have Mosaic 1 for the next 25 years.
UA is going to do what it can to pay other airlines as little as possible for BE seats, so it's definitely a possibility that B6 would follow suit and cut earnings for UA BE fares.