I don't care what they call it. The B fare IS the discount F price. Remember, CO is not your friend. If they thought they could sell out the F cabin without Y and B-ups, they would. Providing the Y and B ups to Elites is no different than offering a targeted discount and it is offered for the same reason any discount is offered - not enough demand at the original/list selling price.

And, since this is about selling out the F cabin at something approaching a market-clearing price, why should CO privilege Plats over Silvers or Golds? Doing so would likely reduce the consumption of Y and B fares by Silvers and Golds. Similarly, not making the upgrade confirmed at time of booking would also depress demand for Y and B fares among Plats. Unless they are certain that they can sell out the F cabin by only offering F fares, then they are likely to leave money on the table.