I think due to the no change fees covid waivers, the 72hr rule was effectively canceled for much of this year anyway... as I've had flights where I made last minute changes inside T-72 such as changing it by a day or rebooking due to the mileage dropping.
I would think it should be pretty easy for DL to monitor and catch those who abuse it by booking and canceling last minute to try to score an upgrade, or having multiple conflicting bookings to different destinations.