"Permanence" is not really a new thing though, and not unique to blockchains. For instance, there is no such thing as RTBF on Wikipedia, and they have an elaborate procedure called RevDel for applying for a permanent deletion of a contribution there, and such an application must meet strict criteria, and requires "prior clear consensus". It is not an automatic "right" based on the contributor's whim or fancy. The point is though, they have a policy and procedure, and the technical capability to effect such an approved request. There seems to be no such facility at all for permanently deleting anything from a blockchain.