It doesn't need to be inherently wasteful. A reverse-biased diode provides a completely unpredictable source of noise while also taking extremely little power.

It doesn't need to be inherently wasteful. A reverse-biased diode provides a completely unpredictable source of noise while also taking extremely little power. Or cranking up the sensitivity of a sensor. Or reading the low bit of an ADC. All of these provide good entropy sources without being wasteful.