I think the bridge into the financial system is the value. In theory, any game could create a way for you to "own" a rare item, and maybe even trade it within the game. But then how do you transfer ownership with a guaranteed exchange that can be cashed for fiat? The exchange would require us to trust each other, and I would paypal you the money? I think the NFT serves not only as the proof of ownership, but the chain of ownership and the escrow as well.