Maybe time to ask the Australians how they managed to sleeve the prongs of their plugs without weakening them too much?
Some would dispute the last bit of that sentence, but there's nothing too fancy to it; simply a matter of making the plastic layer just as thick as necessary (and perhaps choosing a plastic with high dielectric strength so it doesn't need
to be too thick).
AS/NZS 3112 was
revised in 2004 to upgrade the strength test requirements (it was the 2000 issue that mandated the insulation, effective 2005), so I suspect some manufacturers may have initially tried thicker plastic than necessary to save a bit of money on the (more expensive) brass.