I had a similar problem last week. My first flight was delayed by a mechanical. I got a call from Delta and they protected me on a later flight for my connection in ATL because the connection was going to be close. We got to the gate in ATL a couple of minutes before the departure of my original flight. The original flight was delayed and I got to the gate when they were about halfway through boarding. When I tried to board I found out my reservation in FC was cancelled. They hadn't cleared standbys yet so I got a seat in the back. Unfortunately a terrible seat next to someone who hung over into my seat for a 4 hour flight.

Protection on a later flight is all that is needed to proactively take care of customers. There is no reason to cancel the original reservation until after the flight departs unless there are standbys and then it can be when final call for boarding is made and standbys are being boarded.

I find the current procedure totally unacceptable. It would cost Delta nothing to change the procedure and it would save a number of extremely upset customers.