I see no reasons for replacing the existing ductwork with flexible insulated ducting unless the existing ducting has rusted out.
I would just re-insulate with 3” fiberglass wrap if the existing metal ductwork is still in good condition.
Some of the reasons for not replacing with modern insulated flexible ducting.
Cost of replacing existing ducting and installation New flexible metal ducting can be several times more expensive than just re-insulating.
Flexible ducting has no advantage since you already have ducting installed
Existing metal ducting if properly sealed has minimum health risk exposure risk to deteriorate of the insulation.
Flexible ducting will have a slight reduction of airflow.
Flexible ducting are harder to clean
I have the same setup as you described. 50-year-old ductwork in the crawl space covered with 1” fiberglass.
I will be re-wrapping the ductwork with 3” wrapping. Before wrapping the ducts, I will seal All the seams with mastic and the proper duct tape prior to wrapping.
See How to Insulate Ductwork (video) for all the details.