Not so fast. I had that on my recent flight to Israel. Then around 5 hours into the flight the FA wakes me to tell me that there is a religious guy who is complaining about sitting next to a woman and they are going to move him to this seat.
I flew to Israel a couple weeks ago, and across the aisle from me was a couple who had the good fortune of landing an empty seat between them. However sure enough there was a guy trying to work out a seat and that resulted in the couple losing their empty seat. The husband was furious about this “penalization” as he coined it, but the FA told him firmly that he had no rights to the empty seat.
A bit later the FA came back and offered his wife an upgrade to premium to compensate, she attempted to turn down the offer as she apparently wanted to sit near her husband. The feeling was clearly not mutual, he angrily insisted she take it, and was able to enjoy her empty seat for the rest of the flight.
Meanwhile all this resulted in the more religious guy and his wife having a full row to themselves with 2 empty seats. Definitely worked out for them.