What? "Upgrade oxygen across the network?"

First of all, United does not care about people being unable to use upgrade certificates. It's a feature, not a bug: they give them away as a reward for additional sales, and then they don't have to give the purchaser any additional benefit.

Second, if they did, it makes no sense at all to restrict StW to preserve additional upgrades for others. People will still waitlist their expiring points on every single reservation they have; "suck[ing] up the 'upgrade oxygen'". If they allowed StW, people's balances might decrease to the point where there actually were some upgrades available for other people with smaller balances.