You say that, but the only plausible non-substitutable use of bitcoin I've seen is sending money overseas or buying drugs. If its monetary value collapses, I don't see these other uses at all replacing it. It could just die, though more likely some marginal stuff will continue indefinitely trying to recapture lost glory.
NFTs have actually solved a real problem in the art world. I've said it many times before here so you can browse through my comment history to see why.
Spoilers: - saying "but there is fraud" is not an argument against NFTs, because actually an automated solution for NFT fraud is conceivable. And the fact that there is fraud does not take away from the thousands of artists using it legitimately. Plus, authenticating you are buying from a real artist in the NFT space is actually not that hard.
I actually think there is a huge potential to decentralized applications, but this is essentially faith for now so I won't add much more to the discussion.
I don't actually believe there was anything technical there in the value prop for art over simply having a centralized organization attest to purchases.
My feeling is that the odds of anything useful coming out of this are small. After insane money and over a decade, nothing useful has resulted. Perhaps conditions will be right in another decade for something useful to emerge, but at this point it's all scams imo
