milk_manWeve been over this. Scientists claim to know how much of each element the rock, etc started out with. And they assume that level based on current conditions. If you can't see that it's an unreasonable assumption then I don't know what to say to you
Let's look at those assumptions. The assumptions are that (1) all of a particular isotope found in the rock sample is due to the radioactive decay of precursors, which occurs at a known rate, and that (2) none of the isotopes used for dating have been removed from the mineral by geologic processes.
(1) If a mineral forms by rejecting a particular element in its crystal structure (for example, the mineral zircon rejects lead, the final element in the uranium decay chains), any amount of that element found in a mineral sample can be assumed to have been trapped there after being created from radioactive decay. Mineralization can be verified by microscopy.
(2) Minerals used for radiometric dating are
known to survive geologic processes such as erosion or transport. Though I don't know this for certain (I'm not a geologist), I'm confident that many experiments have been performed to show the (in)stability of minerals in the range of possible geochemical environments (reducing/oxidizing, alkaline/acidic, in the presence of this or that, etc). We can further inform the design of these experiments and the interpretation of their results by actually observing these environments.
By these tenants of mineralogy, you can determine the original quantity of material by summing the amounts of the precursor and daughter (because the atom doesn't disappear by radioactive decay, it only changes). So we can in fact know how much was originally there.
If you're invoking an argument along the lines of "those dates are really fucking long ago and there's no way you can know that these minerals weren't affected by some cataclysmic shit between then and now", then fine, go for it. But it comes across as a nihilistic argument to reject evidence that goes against your existing beliefs.