And why can’t the logical conclusion be that either one of them could be the original Riker? Why does the original have to be gone?
Even if every cell in your body is replaced over a 7 years period which they don’t since not all cells actually undergo mitosis or are even replaced through stem cells not every molecule in your body needs to be replaced for that to happen and in fact it doesn’t it would be exceedingly wasteful and impractical.
So the many if not most hydrogen molecules you were born with are still probably somewhere in your body.