The starting isotope is called the parent and the end-product is called the daughter.
It is true that radioisotope decay rates are stable today and are not largely affected by external conditions like change in temperature and pressure, but that does not mean that the rate has always been constant.
Far from being data, these dates are actually interpretations of the data.
As discussed before, the assumptions influence the interpretation of the data.
The method of calculating radiometric dates is like using an hourglass.
You can use the hourglass to tell time if you know several things: the amount of sand in the top of the hourglass when it started flowing, the rate that the sand flows through the hole in the middle, and that the quantity of sand in each chamber has not been tampered with.