I agree that checkpoints in POW fail the trustlessness goal. It seems you also agree that POW/POS are never both append-only and trustless. But based on my reading of the consensus algo of eMunie, its possible to have a ledger system that is all three important qualities: trustless, append-only, and resourceless (meaning doesn't waste resources). Is there another crypto with those three?
The last thing I read (months ago) was that eMunie was trust based, and followed a similar security model to POS, but things may have changed since then. Fusilier is supposed to be writing up his consensus summary when he gets a moment in this thread.