Oh no, the 500's won't go anytime soon. Like you explain, they are only 10-12 years old and they share a lot of common features with the 300's which are operated in large numbers at Southwest. Common features are: identical cockpit and many spare parts in common. At Southwest a pilot can fly all 737 variants but normally as far as the FAA is concerned a pilot who is rated on the 300 is automatically rated on the 400 and 500 with no additional training, but not on the NG series. The 200's (I saw one in FLL two months ago, it was still in the old orange and red livery) will be gone in the mid 2000's but the 300's and 500's will stay longer than that, till at least 2010. I don't think Southwest will buy 800's and 900's because those require an extra flight attendant in the cabin, that's because they have two overwing exits on each side. That's why they never got any 400's, only 300's and 500's as far as 2nd generation 737's are concerned. They are not looking at the 600 because that one is too small, so I think that the only variant they are looking at is the 700.
Southwest is focussing on only one type for the future: the 737-700.