> The key trait of a blockchain is that each new entry cryptographically verifies all previous entries

I see what you're saying. Basically: distributed database + signatures + merkle tree = blockchain.

I usually use this definition: decentralized and sybil-resistant database + signatures + merkle tree = blockchain.

So I think the key trait that turns a distributed database into a blockchain is not the merkle tree, but is its decentralized nature. Anyone can be a transaction validator and no one can prevent transaction validation.