This is a positive for frequent flyers. They board late due to a couple beverages or late transportation and the low priced tickets who brought way more bags than they were allowed all of a sudden jam packs the overhead. Frequent flyer now gets to gate check his bag, delaying the flight possibly. That ****es off people, making the flight attendants unhappy who in turn let us know about their unhappiness.
Or, gate checking and extra bags slow things down enough it possibly messes up connections, so we fly faster burning more fuel hurting the bottom line. If people do happen to miss their connection all of a sudden we might be paying for meal vouchers and hotel rooms and rebooking flights.
Several airlines are already enforcing this carry on baggage policy, charging for it, and if people don't comply they are charged at the gate. Pay or don't go. Follow the policy or don't go.
This is hopefully some job security, making money on the fees and saving money on less weight and hopefully better on time. and it will open overheads.