In some ways this whole industry creates it's own problems because airlines don't want to get pay to get their pilots certified for new planes. So Boeing tried to engineer their way out of the problem and hack in some changes to make an entirely new design fly like an old 737-800. Southwest Airlines was a major contributor to this as the largest purchaser of 737s in the world, because you know, god forbid Southwest's pilots had to get certified to fly a new plane once every 50 years.