Nobody. People are supposed to fold their arms across their chest and wait to die. If/when the plane safely comes to a stop on the ground, people get to get into their cars or taxis and use whatever arm rest they want when they get there. It's a whole, big, master plan aimed at efficiency and a smooth process in the event of all possible outcomes.
Yes, the quandary! I only raised the question as i was ona flight last month and both window and isle were already in use. So i eased into the middle chair to find that both window and isle had taken the liberty of having 2 arm rests. I folded arms, as frustrating as it was:)