> Even if every archival node deleted some old blocks, all that'd mean is you had to start your node from a snapshot of the database taken at the earliest block time and work from there.

Why would every archival node delete the old blocks? Wouldn't it be prudent for at least a significant minority of nodes to keep copies of the entire blockchain in case there is ever any dispute about the provenance of the snapshot?

> Given that outputs which store only files cannot be spent

Is that a hard requirement? There is no feasible way to encode both a file and a legitimate transaction into the same block?