737-900: Cheaper, relatively same size. Unfortunately, aesthetics and the ability to rocket off a runway even with a full pax load aren't very high selling points for new aircraft.
IMO, The shutting down of the 757 line is symbolic of the glory days gone forever. Flying is no longer the experience that it was back in the 50's, 60's, 70's, and 80's. Used to be that flying was done on graceful "Clippers of the sky," or "Luxury Liners," whereas now flying for most people is like getting on a bus. An Airbus.
And all at once the crowd begins to sing: Sometimes the hardest thing and the right thing are the same