OK, I am imagining. That is all you can do with that.
Easy, because the former relationship of the materials in the rock had nothing to do with decay. Since they do now, soon as you see Y, you imagine it came about by decay, over long ages. When actually, we may be better off looking at it as simply so much Y.
You see, a same state past is required for Y to have been in a decay relationship, if the rock is more than 4400 years old. (if that was about the time of the change, as indicated in other evidences, and data, such as Dodwell's curve)
Without a temporary present state universe, and fundamental forces, etc, as they now are, the materials present simply were there in a certain ratio. Half lives did not exist, they are a feature of the decay state. Across the board, or, 'line' if you will, this is true, not just in some particular isotope.
The other thing we may need to consider, depending on evidence, is whether the addition or loss of a neutron, or some other atomic level change, (as a result of the big universe state change) is responsible, or partially responsible for some ratios and materials. As pointed out in the last post, even adding one neutron changes things.
" "The number of neutrons determines the
isotope of an element. For example, the
carbon-12 isotope has 6 protons and 6 neutrons, while the
carbon-14 isotope has 6 protons and 8 neutrons.""