It is unlikely any future high voltage DC system would be sufficiently safe without tracking leakage current as I outlined. Partly because it is almost impossible to stop someone cutting through the insulation, however thick (eg. with a kitchen knife, lawnmower, fire, etc).

3000 volts DC is a "definitely dead" voltage, as opposed to current 110 volt AC systems which are "you'll probably survive" if you use a kitchen knife to cut through the insulation.

Given that you need the protection systems in place anyway, there isn't much point in thicker insulation, unless you like your cord being more cumbersome, heavier and more expensive.