If you read the definition of blockchain by Nakamoto in the post, the most important point of a blockchain is to provably keep track of the order of the transac

If you read the definition of blockchain by Nakamoto in the post, the most important point of a blockchain is to provably keep track of the order of the transactions. That is not so easy to achieve with just a public key infrastructure. Actually, timestamping is a very difficult problem and bitcoin solved it.