There are always airlines, even in first world markets, interested in leasing older A320s as well. Lease rates go down a lot after the first 8 to 10 years. That said, for the leasing company it is probably more profitable to break it down for parts and sell it off.
And of course there are quite a few differences between the first 737-800 to the current ones. Different engines, although they can be swapped around (7B vs 7BE), more capable avionics and split scimitar winglets, to name just a few. Every aircraft evolves over its production life.