[dozzie]

> The key innovation of the blockchain was the use of proof-of-work in order to associate an amount of computer-time with a given block

Correct. In other words, proof of work is only a tool to make everybody slow in producing data while keeping everybody fast in verifying it.

> In short, the blockchain solved the Byzantine Generals’ Problem,

No. It's a common misconception, but in fact it does not solve Byzantine generals problem, with the simplest reason being that the problem requires the solving protocol to terminate (blockchain does no such thing).

> and happened to use a append-only cryptographic ledger as part of it.

The correct terminology is not "ledger", but "document timestamping". You don't need to record transactions in blockchain, they can be any statements. Cryptographers already had such systems, except that each of those required trusted third party to produce timestamps.