Boeing delayed replacing the B737 with a new type as they knew if they asked their big 737 customers to transition to a new type, then those same customer would rightfully take the opportunity to conduct an open market competition as to which aircraft to choose, which would include the Airbus A320/A321. Boeing really didn't want their biggest and most loyal customers to start looking around for the best plane.
Most large airlines have a mixed Boeing/Airbus fleet, so 787 and A350, 737 and A321. This way both manufacturers offer their best prices to keep the business and hopefully steal some from the competitor.
G