So far the isotopes called "stable isotopes" are ones with unobserved decay, and slowly over time ones that used to be called stable are being found to decay (e

So far the isotopes called "stable isotopes" are ones with unobserved decay, and slowly over time ones that used to be called stable are being found to decay (e.g., Bismuth 209). Quantum uncertainty (and still theoretical proton decay, expected to happen, just not measured well yet) are expected to get the rest. Ultimately quantum tunneling will get them all. There is no known mechanism to prevent it.