>>> Then some poor SOB on standby came on board.
One proper way of dealing with the situation is for the flight crew to put off a standby. (The standby may or may not be given the choice of squeezing into the half-seat.)
Another proper method is to have the larger person sit on the edge of the seat row and into the aisle. This requires that the armrest at the aisle be movable.
>>> you just need to deal with it
It's the airline's duty to deal with it. Compensation is yet another way.