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Indeed, nothing in science is ever "proven" beyond all possible doubt; there is no way of knowing, with 100% certainty, that one's proof is foolproof.
Unfortunately, Mother Nature does not give little, gold ribbons to certify the accuracy of our proofs!
Billions of years from now the depletion of the sun's hydrogen will upset the sun's internal balance, and the sun will again undergo some long term changes.
But, that has absolutely nothing to do with the shrinking-sun argument above, which attempts to prove that the solar system is less than 5 million years old.
Great care is taken to survey all the relevant literature and to arrive at a balanced judgment of the known facts.
Scientists are trained to overcome a one-shot, "cowboy" mentality.