Grandfathering is based on the historical experience and in-situ demonstration of adequate safety. It is obviously measured against the initial design means of meeting the regs (in this case uncontained engine shrapnel) Any significant new experience across the total fleet s of all similar aircraft designs, beyond the B37 Max fleet, may be considered and as such "Special Condition" in the design required by the regulator.
Up to the MAX the engines were under the wings and therefore the rudder cables were effectively protected by the wing centre structure and therefore are compliant with the regulations at time of certification. In the MAX the engines are a new design so previous engine fail rates are irrelevant and they are now ahead of the wings so they plainly don't meet the regulations as specifically stated by Boeing themselves. There are obvious and proven mitigation strategies to eliminate the danger to the rudder cables, yet Boeing elected not to employ them for purely commercial reason with the acquiescence of the FAA.
Grandfathering by itself is not inherently bad, however I think there is an obvious requirement to fully assess the impact of changes to the baseline airframe and mitigate any new dangers. So far it appears that Boeing failed to do this with
1) The MCAS
2) Manual Trim wheel effectiveness at high air speeds
3) Rudder cable vulnerability to uncontained engine failures
This is not confidence inspiring........