I really think UA is missing a trick here. They should adopt the NOR-1 standby upgrade system like Hilton has to manage TODs. "If the seat is available, do you want to upgrade for $XXX?" Then offer a higher upgrade fee if they want a confirmed upgrade instead.

That would probably leave more last minute room in F for high-priced Y tickets, while still capturing paid upgrades if tickets such as the type you buy don't get purchased.