Just because they'll have to fly two types eventually doesn't mean they want to spend on making the switch now, especially not with an airframe that is an incremental improvement in fuel economy. (Not to mention that other OEM offerings suddenly look a lot more viable for operators wanting newer generation narrowbodies if they're forced to change type certificates regardless)
Genuinely new types that might offer a step change in operating economics that's worth all the hassle carry the risk of more aircraft programme delays and early operational issues anyway.
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.
Genuinely new types that might offer a step change in operating economics that's worth all the hassle carry the risk of more aircraft programme delays and early operational issues anyway.