And Boeing's original plan was to do a clean-sheet redesign, but Airbus launched the A320neo which within six months had over 1k orders, and Boeing felt commercial pressure from its customers to launch something comparable (a re-engining, not a clean-sheet design) so its customers weren't being undercut by their competitors (who would then have lower fuel costs, etc.).

Airbus notably have recently been hiring for a clean-sheet A320 successor (though there's no firm commitment to that, yet), despite it being a 20 year newer design than the 737.