Energy wastage in wireless charging rotary toothbrushes?
I can't give the detailed working information about how a toothbrush holder delivers power because I don't know that but I can tell you that theoretically no power has to be consumed if there is nothing sapping power from the magnetic field.

Think about a coil of wire attached to a sinusoidal voltage source. If the coil is made from low resistance wire and is reasonably "reactive" to the frequency of the sinewave, the current drawn will be near enough 90 degrees to the voltage waveform i.e. it will have a power factor of virtually zero and very little power will be used up.

As the toothbrush (or whatever) comes up close its coil can convert some of that magnetic field into electrical energy using the principle of induction. Therefore real power can be withdrawn from the mag field and the phase angle that was 90 degrees becomes somewhat less. The power factor is no longer zero and this signifies power is taken from the voltage source and delivered to the toothbrush. It does look like the toothbrush uses AC power frequency like this: -



Operating at a much higher frequency is quite possibly a sensible thing to do and this is exactly what passive RFID tags utilize. A lot operate at 13 MHz and you can make quite small coils and, to get extra distance they are resonant tuned with capacitors. All done without incurring hardly any off-load power: -



I've built bigger ones to supply energy to telemetry electronics mounted on a spinning rotor in a turbine.