The answer to the stage of a blockchain you mention would seem to be, "If the blockchain isn't distributed and scaled enough to be resistant to a 51% attack, is there really any value remaining in it?"
You wouldn't think today's newspaper is interesting, but to someone doing research, the newspaper of a hundred years ago is interesting.
You wouldn't think someone's Geocities site was interesting, but when that shut down, the Internet Archive spent a lot of energy saving what they could, and that's already interesting for someone researching what the Internet was like a mere twenty years ago.
Blockchains get sold as these wonderful systems for storing data and also coincidentally making money off of ICOs. If they're going to actually be used to store real, useful data, then even if there stops being am immediate use for it, future historians may find one.
Nowadays many of these ICO projects are vanished: websites are down, "whitepapers" are gone, team members are in jail... Somebody in the future might want to reason about this phenomenon. As this was mostly an internet thing all data and information lived on the internet only. I think even today it might already be difficult to research about this, as lot of information is already gone.
There is little to gain from attacking a system _that_ unpopular, but you could "take" everybody's coins in that scenario.
This is actually the failure mode of most altcoins in the wild. They get attacked because the consensus was never reliable.
There are still tons out there (easily more than 50% by number) that could be smashed into oblivion by anyone with a decent amount of money tomorrow.
I'd imagine such a record to be: the final block header, and a Merkle tree root hash of every block using a more time-resistant hashing function. The Merkle tree root hash prevents rewriting the chain later through brute force. The actual consensus mechanism has prevented wrong writes.
Timestamping could be done by publishing in newspapers, or in other blockchains.
The biggest issue comes at the moment 'archiving' is announced. History-rewrite attacks then suddenly become a lot more valuable, so you'd probably need to say 'We are archiving the chain as of 100 blocks ago'. This prevents anyone from mucking with the end of the chain, but comes at the cost of discarding the last 100 blocks.
2) if there is utility in the blockchain, chances are it won’t shut down.
This is incorrect. Digital signatures are used when (S) signing a transaction which must refer some precious transaction with a receiver address corresponding to the public key which verifies signature S.
Blocks are chained by their headers hashes. There are no keys nor digital signatures involved here..
To call the solution to a block a 'signature' is not incorrect.
(personal opinion): On the other hand, I don't understand why it's called a "signature". Pub-key signs are used to prove integrity of some information and some form of authenticity related to the priv-keys. Block header hashes are related to integrity of immutable info, but are not related, in any way, to authenticity. So I still don't find it obvious, nor that it's appropriate, that it's a "signature".
