What happened to the Boeing 737.X design?
Boeing was considering designing an all-new airplane to replace the 737 around 2020. Having been in production for over 40 years and through 3 major versions, it seemed like it was finally time to leave the 737 type behind.

However, Airbus decided to upgrade the main competition to the 737 to create the A320neo. The A320neo is more efficient than the 737NG and will be entering service in 2016. Boeing watched orders accumulate for the A320neo and felt that they would lose too much market share in the single aisle market if they waited until 2020 to introduce a new plane to compete with the A320neo.

The high number of orders for the A320neo convinced Boeing to scrap the new plane concept, and focus on just new engines for the 737, in order to get a plane on the market to compete with the A320neo. This allowed the 737MAX to be scheduled to enter service in 2017, less than 2 years after the A320neo. While the MAX does not have as many improvements as a new design would have, it has helped narrow the gap in orders to the A320 series. A new plane still 5 or 6 years away while the A320neo enters service would not have nearly as many orders, meaning the Airbus would be quickly increasing their market share with the A320neo.

A similar situation has happened with the 777, although it is still a much more recent design. The 777X is a more significant redesign than the 737MAX, but the update is needed to compete with the new A350 that is already in service.