I said hysteresis loss, but that's just the first barrier. Higher frequencies are also subject to the "skin effect" which requires thinner wires, therefore more wires or ones with lower resistivity (e.g., superconductors). Then you need to get transistors capable of handling the increased switching frequency, and you need to be able to deliver the increased drive current to the transistors.
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.
