Grandfathering is not unique to aviation and you are welcome to research this.
Grandfathering is not unique to commercial aviation. For example, it's common for a ship or building constructed in 1990, to be grandfathered to the applicable maritime or construction regulations current in 1990 (or when approval was granted), exempt from new rules being retrospectively applied.
In such examples, grandfathering can be lost. For example if modifications proposed are deemed to be significant.
What is unique to aviation, is the ability to design a new aircraft model, incorporating grandfathered conditions applicable to earlier models, equivalent to an architect wanting to build an identical building next to the 1990 constructed building today, to 1990 standards, using grandfathering arguments.
Grandfathering has become incredibly fragmented, to the point a 2017 build aircraft, is a composite of components and sub-structures from differing standards and dates. Could there me incompatibilities between such interconnections that haven't been fully explored, and are weak links?
Grandfathering is incredibly valuable, both in time and dollars, to Airbus and Boeing. So valuable, they each employ a team of engineers and lobbyists, many recruited from airworthiness authorities, to counter / pre-empt arguments put forward by airworthiness authorities.
There was (may still be) a loose alliance of other commercial aircraft manufacturers, including representatives from Russia, China, and Japan, that had dialogue with airworthiness authorities and WTO, to limit / abolish grandfathering, which creates artificial trade barriers and costs to clean sheet designs versus those with grandfathering.
This group also lobbied the insurance and finance industries to view grandfathered commercial aircraft as higher risk. There will only need to be one major accident where grandfathering is in part or totally attributable (or grandfathering mix and match), for premiums and funding costs adjusted to adjusted to bring the practice into line with other industries.