First of all, I'm not trying to say that there isn't a single use case for blockchains out there. I'm also not going to pretend to understand blockchains beyond a hand wavy high level understanding, so before we go on, I'd like to outline how I understand blockchains work, so we can at least agree that my understanding isn't fundamentally flawed:

1. Blockchains are kept "authentic" by calculating a set of hashes which are updated each time a transaction occurs.

2. All past transactions have an effect on the current hash, therefore you cannot modify any prior transactions without also affecting the hash, making it obvious if someone has tampered with it.

3. This is only valid if there is more than one copy of the blockchain (or parts of the chain, I'm sure nobody is sitting there with the entire Bitcoin chain, it's probably huge), because otherwise the hash changing is meaningless, nobody would be able to tell if it was tampered with or not as you could just recalculate the whole thing.

4. A higher number of users (and part holders) of the chain increases the security of the chain, because if there were, for example, only three users, two of them could agree to both make the same false transaction, and validate each other. The consensus then goes in their favour.

5. Because the entire chain is in a way involved in calculating the hash for the latest transaction, the cost of calculating it increases over time.

I'm not trying to write a scientific paper here, so I'm sure there are incorrect details, but are we broadly in agreement?