op-ups usually happen at the last minute. I dont think ive ever heard of someone being so gracious to offer their Y seat in exchange for a BF ever actaully getting the op-up.

if you are using the CO seat map, keep in mind that certain seats and/or rows are blocked out and appear assigned. that means, there is possibly 10+ unsold seats. CO also oversells most flights using statistical data showing that a certain number of PAX do not show for that flight.

and if the flight is 100% full in Y (and all paid pax have been accomodated and boarded and mileage upgrades processed), but BF still has three seats open in BF, CO employees will get those seats before random op-ups.