This is similar to a stall in a processor pipeline. We want to avoid stalls, which mean that we want people exiting the plane constantly. A column approach would work much better: people in the aisle get their bags before the doors open, and start leaving as soon as it does. As columns drain, the next column can get their bags and start to leave. This approach keeps people leaving constantly, while also keep the aisle constantly populated. Yes, the person in the last column in the back row still leaves last, but I claim they will leave sooner. (edit: Thinking about it more, I actually think the people in the last column in the front row leave last. If you're in the back row, second column, you can stand up as soon as the person in the first column in front of you has started walking out. After the first column to leave, people will exit in reverse order because the openings will appears back-to-front. Trying to maintain front-row "fairness" will just result in a period of time where no one is in the aisle.)
The reason we do the row-by-row method, I think, is that our sense of fairness is influenced by who we're looking at and proximity. We look forward, and we feel that the people closer to the door "should" leave first because they're closer, and we're looking at them, so we feel bad if we hold them up. But by doing the row-by-row method, we're holding up everyone behind us, but we don't look at them as much.
I don't know how to enforce a column-by-column exit. Airlines can enforce how we get on the plane because they control, person at a time, who enters the plane. How we exit the plane is more cultural, and while an airline could certainly try to ask people to exit this way, it's much harder to make it happen.