It wouldn't take much to change it at the Amex if the law changed. The authorities have to be honest with themselves, thousands and thousands of fans stand in seated areas all over the country at many top level games. I saw a game at Bayern Munichs old Olympic Stadium and each person had a designated spot on the terrace as if a seat was there. Each row had a crash barrier to lean on, and it could have seats slotted in if required. It was very organised, as easy to police as seating, and very enjoyable for the spectator.
If this is the case there probably wouldn't be any increase in capacity and subsequent reduction in admission costs. Would people be willing to pay the same price to stand as to sit down?