The problem is that 737 design requires significant redoing to make it inherently safer (either structural redesign, or switch to Fly-By-Wire), with all options

The problem is that 737 design requires significant redoing to make it inherently safer (either structural redesign, or switch to Fly-By-Wire), with all options involving significant certification hurdles. At some point the question becomes if there aren't bigger wins possible with new airframe. Or at least resurrecting 757 and maybe making a shortened version.