Whenever any cryptocurrency-related thing seems useful, you're seeing the potential of the underlying cryptography. Not the potential of the cryptocurrency thing.
Everything after that phrase does require the blockchain though- not individually, but as a sum it would be very difficult to bring that all together in a centralized manner.
Nope; you just need a P2P network. Exhibit A: BitTorrent. (Surprise surprise, the blockchain also requires an underlying P2P network.)
A blockchain is actually more centralised than the underlying distributed network; it introduces centralisation-related problems that wouldn't otherwise exist.
Yeah I agree. Though seems pedantic to imply blockchains aren't useful because they're "just" composed of cryptographic primitives. Shared state under potentially-adversarial conditions isn't an easy thing to achieve, and has usecases for which other P2P networks aren't sufficient.
I don't think they're useful to most people right now (or maybe ever), but there are some cool things in DeFi that are normally reserved for large institutions/not possible in traditional finance:
That's hashes you're thinking of. NFTs require hashes, but hashes don't require NFTs.
Whenever any cryptocurrency-related thing seems useful, you're seeing the potential of the underlying cryptography. Not the potential of the cryptocurrency thing.