I think there is a notion (certainly not here on FT) that a seat that is bought and paid for, yet not occupied, should remain at the disposal of the passenger who paid the ticket. You paid for passage rather than a seat, so it's not yours to keep in the event that the other passenger does not travel.

I don't want to troll this thread but I do have a doubt that is somewhat related to this theme.

If a passenger is travelling with a companion in two separate tickets, and one of them is upgraded due to status and the other is not (and the system does not register them as a couple), can the PAX who is upgraded gift their seat to their companion and take the companion's seat instead; or does that fall into the same category as the situation above, and the airline would take back the upgrade seat and use it for another status passenger?