They should've kept airframe and hydraulics (99% of them, just a touch here and there)
DO you have any idea of how much of the 737 does not meet the modern day safety standards and is grandfathered? That's probably worthy of a thread of its own.
Nearly 25 years ago Airbus complained to JAA about Boeing's grandfather rights that were letting the 737 get away with nearly half a dozen more seats than a comparable newly certified airliner could, all being equal.