From an ethical perspective, I agree with you. What I'm wondering is if Delta has any LEGAL obligation to keep you in an upgraded class once you've confirmed it?

I guess I could understand that there might be very rare instances where Delta would have a problem with an overbooked flight. If that had happened to me, I'd ask them to either confirm me in business class on the flight I booked, or confirm me in business class on another flight (even if on another airline) in business class.