First class on domestic flights isn't very good; you just don't get your money's worth. But that isn't the author's complaint.

I suspect the motivation for changing all of the planes is simplicity in maintenance and operation. As equipment obsolesces, finding and retaining people who can fly and maintain it probably becomes difficult. Being able to swap planes and crew members (air and ground), especially in times of a labor shortage, seems to me very valuable to an airline.