I also don’t get it. Auditable trail? Use a Merkle DAG. Or something like this Permissionless Timestamping Network: https://intercoin.pdf/whitepaper.pdf
I mean how hard is it? What are the real challenges?
Why wouldn't you put it all in one spot? The technical requirement of needing an audit table is ubiquitous and I bet almost every SQL database in history eventually ends up having a history table of some sort. This technical requirement is unrelated to any regulatory requirements about (not) storing data in the cloud, and certainly shouldn't be coupled to it.