I think the blockchains need to be designed to be immutable with the exception of allowing them to be split. As long as there are periodic checkpoints within them which hold the current state of the blockchain, there would be no burden of re-indexing it all frequently, which is currently a big challenge for the Steem blockchain. With correct design, the tail(s) can be archived and validated separately and needn't really be a burden. A social media site doesn't suit this approach quite so well because people want to see the older content, so that content should really be in a separate data-store instead of being integral to the blockchain. There's also a governance issue, that this would address.

I hope to write more about this in a post soon.