Thaumaturge is a class which deals with death and LIFE. Whitemage makes sense as a subset or specialisation of THM which deals with the LIFE component of Thaumatury, allowing for the possibility of another job to come along in the future which specialises in the death component.

It's easier to reshuffle abilities than to restructure the entire lore of the game which is what people want when they say WHM should belong to CON.

I guess it could go both ways, but THM is the most obvious ancestor to WHM.