Your 20% figure is not even close, IMHO. Provided that an interchangeable Humira FoB hits the market around the same time as non-interchangeable FoB’s, the non-interchangeable products will likely be non-factors in the marketplace and the sole interchangeable product could take half of the overall volume.
If price erosion for the brand and the interchangeable FoB comes to 20% and the gross margin relative to the monopoly brand is reduced accordingly, the net profit for the sole interchangeable FoB is at least 35% of the pre-existing profit for the monopoly brand.
“The efficient-market hypothesis may be
the foremost piece of B.S. ever promulgated
in any area of human knowledge!”