Nobody can forsee the future or guarantee that all of the underlying assumptions are correct.
Current scientific theory on Earth age is based on assumptions that radioactive decay rates have remained relatively constant. Their observations tend to support this, as they have never been observed to be substantially variable.
The only YEC theories I have seen that attempt to account for the observed data on uranium decay posit that by some yet unexplained process what appears to be 4.5 billion years worth of uranium decay happened within the space of about a year during the Noachian flood, and happened without the ususal heat that would be associated with the decay. Otherwise the amount of heat that would be generated by that amount of decay in that time frame would have vaporized the Earth.
You are correct and I have pointed out that fact on a number of YEC threads. I either get the online equivalent of a blank stare or someone tells me that other physical properties, like Planck's constant, the speed of light, and the mass of atomic particles, have also decayed proportionately. Thus E=mc^2 is not violated.
Despite the fact that I have degrees in chemistry and chemical engineering from major universities and a lifetime science career, I'm just too dumb and too much of an atheist for not buying into that reasoning.
In a miraculous coincidence, all those constants assumed steady state around 1960, which also happens to be about the time when our technology was advanced enough to perform extremely accurate and repeatable measurements. One YEC pointed out to me that, according to the latest work of the great scientist Barry Setterfield, the speed of light and other constants are now in an oscillating state. Unfortunately, experiments don't reflect this revolutionary new discovery.