If the data originates offline - e.g. paperwork for an aircraft part - you still need to trust whoever puts the data on the blockchain. Are they who they say they are? Are the documents trustworthy? The blockchain ensures tracability after data is on the blockchain, but not that the initial data makes sense.

So if you need to verify these aspects manually at some point, might as well have everybody work through a regular API.

However, in case of a digital currency - e.g. bitcoin - everything is generated on-chain. There is no crossing the offline/blockchain boundary. You can verify that data was generated according to the algorithm, and how it was used afterwards. This is where a blockchain is not necessarily replacable with a simple REST API.

Where market participants can agree on a trusted intermediary for bookkeeping, blockchain really doesn't have much to offer.

I'm not sure if that is entirely true though. The blockchain piece might provide the needed infrastructure, while providing needed authentication. I'm not really sure though to be honest. Maybe someone with more insight can point out whether the blockchain piece is really needed and helpful or just bloat.