"...you can never secure more things on Bitcoin than bitcoins."

Well, that's not true, although the means of inserting data into the ledger is not one specifically stated as intended in the white paper. Various DLTs are now being used as the basis for data storage and exchange, such as Tether being used by Blockchan, and keys for deadmen in... er, I'd better not say.

A small transaction is required, but the keeping of extraneous data is rapidly becoming a feature of DLTs.

Isn't soft consensus more like doing double entry bookkeeping without the difficulty of undertaking doing inventory at the same time?

Thanks!