I think UA will take the cash from anyone regardless of their status. I do not think they will drop someone's confirmed upgrade, but will offer the upgrade for sale before they will confirm a gate SWU. And as to your comment :
"t seem in the long run that they would want to make the guy who flies 100K EVERY year happy, before making the guy who only flies 25K per year happy?" when did UA care about making the best customers (or anyone) happy! Sorry, that caused a chuckle from me!!!