I'm not going to argue the pros and cons of overbooking, though there are FAR more pros than cons.

What you are talking about is completely irrelevant to this discussion. There is no indication this was an overbooking situation. There is no indication that the airline "double dipped" this seat. The only thing I've seen that suggests there was overbooking is the description of the video itself by the father. In fact, one could argue the only double dipping that occurred was by family itself who purchased two tickets for the same route on the same day with the intention of only using one of them. How that managed to avoid a red flag from Delta, I do not know. Perhaps it was on another airline.

The ticketed passenger for this flight failed to appear for the flight, they no longer hold the right to that seat. It says so in the terms and conditions for every airline ticket sold on every airline everywhere in the world. If you don't show up by XX minutes prior to departure, your seat is forfeited.

Your baseball analogy holds no weight as people show up to sporting events at a wide variety of times, including two or three innings late (ever been to a Dodger game?). The team isn't going to get much use out of reselling a ticket for the final five innings of a game because only two of three ticketed fans showed up. Everyone has to be to the airport for a flight by the exact same time, says so everywhere you look from purchasing the ticket to arriving at the airport.

Do you really want airlines to not be able to accommodate standby passengers or an IRROP passenger who may have missed a previous flight due to weather or mechanical? They are able to do that based on the assumption that people aren't going to show up and they will have seats to give away.