“…the commercial reality of this kind of thing means that a large number of trilogies (or more) die after the second book, because the normal trajectory of a series to lose readers with every installment, leading to a death spiral.”
There was an interview with Chris Claremont in the Comics Journal back in the early nineties where he discussed the way that publishers helped reinforce that spiral - book one, they print X copies, sell X minus Y copies as planned, book two, print X minus Y minus Z copies, and you’re already to the point where you are incapable of recouping any royalties.
