One thing that hasn't really been mentioned here yet but probably accounts for some of the difference of opinion is the 401K at SWA vs the A fund/B fund system at most majors.
I'm still on the outside looking in, but from how I understand most majors pay based on plane and position, thus a 747/777 captain at 12+ years makes the highest pay rate. Since A funds are usually based on some percentage of your final 3 years pay, at a major with traditional retirement, if they extend mandatory retirement to 65, then everyone has to wait 5 more years to get those big ticket, high pay years in the widebodies that will determine their retirement forevermore.
Whereas SWA is only a 401K and only one payscale/aircraft type. So, there is no need to stay and max out your last 3 years, if you get enough in your 401K, you can call in rich whenever. Conversely, if you underfund it or make very aggressive (unwise?) investment choices, you might really, really want those extra 5 years. And at SWA, for the most part, it doesn't really hurt the guys below. At DAL, let's say, there are a finite number of 777 Captain seats, people have to retire to free them up. At SWA, at any one time half the pilots will be captains, and once you have 12 years in, you're at the top payscale already.
About the growth vs. retirement, it seems that if SWA is really going to get another 400 planes by 2012 (number isn't exact but a guess from old info) they will pretty much double in the next 10 years and most pilot list growth will be from expansion anyway.
I looked up the airccraft orders, from the Airinc article in July 2002. they have 132 firm orders, 87 options, and 217 purchase rights for a total of 436 between now and 2012. Obviously anything can happen, but those were the plans.
Finally, one more aside. I remember from an Airinc seminar (I know but they do put out some really good info at times) where this question was asked and the speaker pointed to the B funds as a sticking point. Apparently B funds were designed as a way to carry airline pilots from 60 to 65 (social security age) since federal law prohibited airline pilots from working during those years. Presumably if the age 60 rule goes away, so will the justification for B funds, which can be quite a considerable sum. Another reason why SWA pilots might have a different take than most ALPA carriers, since SWA has no B fund.