Good point. A servo does continue to measure it's actual position compaired to it's commanded position and applies motor voltage in the proper polarity and amplitude to correct for the positioning error.
Only if the PPM signal and power are applied...
When they are not applied, though, a servo can still "hold", because a servo's (RC) gear reduction is accomplished using a spur gear drive train. Spur gears, when there are enough involved in a gear reduction system, are difficult (but not impossible!) to backdrive; it can take a lot of torque to do so on the output shaft (so much so that the shaft or teeth on the gear can break).
I do agree, though, that if you need an "impossible to backdrive" transmission that can stay that way with power off, the easiest way would be a worm-gear drive gear motor. Barring that, a mechanical solution using an electromagnetic brake (that is applied only when power is off) is the next best thing.