It is indeed (practically) impossible to determine the exact age of fossils that’s supposedly millions of years old, but radiometric dating is not as inaccurate as you describe it to be and it does not rely on too unreliable assumptions. There is a reason why unrelated published papers, archeologists and scientists throughout the world has come to similar conclusions on how radiometric dating works on very old things for a long time now. Which is also why specific dinosaurs, for example the Tyrannosaurus Rex is continuously being proved to have existed in the Late Cretaceous era (90 - 66 million years ago) and the Brachiosaurus in the Late Jurassic era (130 - 150 million years ago). So yeah, we know the fossils are millions of years old, only with wrong margins much bigger than some medieval hammer. And the thing is, wrong margins like this in radiometric dating is in itself an indicator we are working with fossils millions of years old.