First time I am hearing that PS's previous gen console will have great legs after a new gen release. Are you sure you are not making this social conventional wisdom up?
With backwards compatibility, all the more reason people won't fork out 200-400USD for last gen's consoles.
PS2 is a special console. It sold 155million units for a reason. It's got great legs for a few reasons. It had a huge library of games which was exclusive and many people still wanted to play those games, it can play conterfeit games. Basically it bought on a lot of value to the consumer, so I am not surprised if it had great legs.