Yeah, if there was verbiage in there that said you could convert tickets to companion for a refund or even a non-expiring credit after earning the pass, I would have done it because I would have guaranteed saving at least what it cost, minus the time value of course. But that would not be in their best financial interest, so I get why they would never allow that.
It could be an interesting play on their part to allow that option at some point in the future, but make the companion pass more difficult/expensive to achieve. There would be some portion of people who signed up for the deal who would then fail to qualify, and Southwest would get to keep all the revenue from the tickets already booked that were planned to be companion fares by the customer. It wouldn't be unfair on SW part per se, because the customer would still get to use all the airfare they bought, they just wouldn't get half of it for "free."