A new type rating cannot use grandfather rights. There is plenty of stuff in this plane that is not permitted any more (like the door design over the wings). Those would be massive changes to the plane.
That’s just one example though and you’re likely right. The thing is though most of these systems have modern adaptations that could be applied. They just haven’t been because of the desire to keep it a 737. They’re things that can be changed/fixed whereas the aerodynamics of the design cannot be be changed easily.
Oddly enough that news bulletin says it's still a plug door which it clearly is not in the final design.
Overall the root cause I believe is generally weight of doors and evacuation time. I had a few conversations about this around the time the second accident happened and grandfather rights were brought up by people from the industry I talked to as an expensive problem for a new type rating.
"It is the only modern Boeing jet without an electronic alert system that explains what is malfunctioning and how to resolve it. Instead pilots have to check a manual."