That would add flexibility to price different awards, and might be a way to deal with problems associated with capacity controls, but I see a couple of big problems -

First, WN traditionally benefits by having a spoilage factor; we hear so much about other programs having way too many miles on the books (although, unlike currency, they can adjust the value at any time by changing the awards structure). Moreover, as people keep building miles they can't redeem, it just makes things worse.

Of course WN is better off if you never use the award, but if you look at how low some of the costs airlines cite when accounting for the actual cost of transporting someone on a domestic rewards ticket, it looks like $50 could be close to that cost along with any administrative burden in re-issuing. Thus, even if you extend an expired ticket, Southwest makes up for carrying the free ticket for another year. I'd guess it's not well advertised because it still sounds like a pretty manual process and they're not set up to handle the burden of too many of these.

Second, there's the simplicity issue - especially since they've had to push the "live in a world without miles" since "pick a seat, any seat" obviously doesn't work anymore. Credits could start to look like miles. Maybe call them "choices" - oh wait, right credit card partner, wrong program. Keeping it simple seems to be less and less valued (or possible?) these days, but it's still a valuable WN atribute.

I could see some sort of a hybrid situation - once you reach the standard level (16 credits), a ticket automatically gets kicked out to you with the expiration, but there could also be some last-minute or short-haul awards that you could go online and redeem. This also might be of interest for those who still have a hard time getting 16 w/in 2 years - although that's not really the crowd you want to target, I guess. Still, I don't see anything that makes credits valid forever (or until you cash them in).