My question is, what can those ETH wallet be used for?
Greater eligibility potential of getting some token rewards in the ecosystem or new coins on other chains without much effort. But you shouldn't add any assets to that address.
As for a collectible, that's bullshit I think. I don't know exactly how it plays. What is clear is that the previous owner still has the private key, and can sell it several times to other people.
Is that true? As far as I know, I still haven't heard where the Ethereum genesis ICO wallet will get rewards in other ecosystems/chains without much effort. This seems like nonsense, because what I know is that those who have more interactions have more opportunities.
Obviously nonsense for collectibles, remember never be fooled it's possible that he is making money by selling his ETH genesis wallet and it is still possible to resell it to someone else.