“There is no way one can assume the “daughter atoms” were all produced by decay.
Since you can’t assume they were all produced by decay, you cannot measure the original amount of the “parent atoms”.”
-—Some of the radiometric dating methods rely on daughter elements which have sister isotopes. For example, the rubidium-strontium dating method measures the ratio of rubidium87 to strontium87. Strontium87 has 3 sister isotopes of Sr86, Sr84, and Sr88. No other decay process produces Sr86, Sr84, or Sr88 and there’s no chemical way to separate the isotopes or to add one of the isotopes preferentially over the others - thus those 3 isotopes are ALWAYS found in the same ratio - BUT there is often extra Sr87, which is produced by Rb87.
Exactly my point. How do you know that the amount of Sr87 was in the original sample, and not added there later?
With the (essentially) liquid flow of our crust, there is no way to determine the original amount of the parent in the rock. Erosion, plate tectonics, volcanoes, etc all play a part in the composition of rock. There is no way to accurately assume the original composition of any sample.