[mediocregopher]

As you say, only the tail (or really, the head, if you think in terms of cons-ing onto a linked list) needs to be present for a client to operate at almost full operation. The only entities that really NEED the full chain are those who are doing historical accounting, like the various blockchain explorers and those who run analytics. These people are already putting some investment into scaling and processing concerns anyway, so I'm not worried about them.