In 1990 when Diamond Plate was introduced, polyurethane coating technology was in its infancy. They were only just starting to use it on cars too. It was certainly an improvement over the enamel clear coats that were used on games like High Speed, Pinbot and Cyclone... But not by leaps and bounds. Now in the age of Rhino coatings and advanced epoxy coatings, Mylar is rarely necessary.
People even today, still have this misconception that Diamond Plate is forever and indestructible... Its not, even back then, Williams only claimed that it should add an additional 2-3 years of protection (over enamel clearcoat) before significant visible wear occurs in a routed game. This is why even in DP games, Mylar was still used in rebound areas like pop bumpers, slings and high abuse areas like ball drops.
At the same time, my T2 is Diamond Plated, and is HUO... 25 years later, absolutely no sign of visible wear to the clearcoat.