Repeated observatoins and consistent results. The same thing that's produced every other accepted physical constant that's used in scientific and engineering calculations.
Unfortunately, those observations and results have not been performed under the radiometric decay conditions that you are extrapolating.
And again, dynamic time is different from radiometric time, significantly so in this scenario, and you are equating the two. That is why I referred you to post #27.
Finally, the ZPE is probably involved in radiometric decay energies and is certainly involved in radiometric decay rates.