You don't need superconductors to get around the skin effect. It's actually interesting that you mention superconductors, as the skin depth in a perfect conductor is actually zero. (Anyone know what happens at DC in theory in a perfect conductor? I originally thought it was zero at DC as well, but to me skin depth seems like it becomes undefined.)
Even without superconductors, at 100 MHz the skin depth is still around 2 thousandths of an inch. Litz helps too. High power RF transistors exist. All the things you mention are limiting factors, but I still say the lack of good magnetic materials is the dominant barrier to higher frequency converters.
Even without superconductors, at 100 MHz the skin depth is still around 2 thousandths of an inch. Litz helps too. High power RF transistors exist. All the things you mention are limiting factors, but I still say the lack of good magnetic materials is the dominant barrier to higher frequency converters.