The 737 replacement was started and then shelved over 10 years ago. The 800 was supposed to be the final model, and then restructuring issues, the GFC and a few other things had the project canned. Boeing can't do crud in secret when it comes to new designs. Biggest mistake was letting the 757 die, now the gap it filled is getting progressively owned by the A320/321. All because US airlines didn't like narrow-bodies. Boeing came up with the original cross crewed 757/767 idea, it was just 20 years too early, Airbus made it work at the right time.