I can understand the regulations being applied to a type in regular passenger use, a GPWS would have prevented a Metro 3 from flying into mountains near Lockard River in Australia and killing all its occupants.
Surely a DC3 wouldn't be used in scheduled pax service again anywhere in the world, even darkest Africa has relatively modern turboprops.
A bunch of aviation enthusiasts having a weekend jolly to experience a bit of history could be warned at the time of booking that the aircraft doesn't meet modern safety standards and they fly at their own risk.
Next we'll have E Type Jaguars being pulled off the road to have airbags fitted.
I still have DC3 on my licence, but I don't expect to get a job flying one again.