Businesses got caught out when DL rolled out its E fares. The nuance of fare rules escapes people who do not travel frequently and the people who make policy do not travel a great deal. But, it is unlikely that this will repeat itself.
I also suspect that UA is carefully warning its corporate customers to avoid the brand damage DL did to itself.
All of this ensures one thing. People without status sitting in the cheap seats won't take up the limited OH space, even if you are on a tight connection. No matter what the rules and no matter how well-enforced, the average narrow-body can't acommodate the # of compliant carryons. So, somebody loses. Who should that be?