They could, but it would mean that a lot of the supporting infrastructure at airports would have to change, essentially negating much of the advantage from avoiding a new clean-sheet design.
As long as it remains sufficiently 737-shaped airlines can keep flying it to all the place they've been flying 737s for decades.
As long as it remains sufficiently 737-shaped airlines can keep flying it to all the place they've been flying 737s for decades.