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> what is the "correct" chain may be a highly contentious subject

But this can be true in POW blockchains as well. If the user disconnects his node for some arbitrarily long period of time during which there are durable forks created, said user also has to "reconnect manually to the correct chain" and the user may have to decide for him/herself what constitutes a valid chain under contentious circumstances. The user's preferred chain may have become a minority fork, for example.

In the case of the bitcoin blockchain, I think there is an extremely simple protocol: just use the longest (most work) chain among the various contenders for "correct".

That's fine if you simply want to find the chain with the most proof-of-work behind it and join that herd, but what if that chain violates your conditions for validity - perhaps it has different inflation, less anonymity, or higher transaction costs than the minority fork that you were originally following as "valid."

In POW the onus is on everyone who runs a node or miner to understand the consensus rules they are choosing, and to be on their toes in the event of a fork (hard or soft). I see this as no different from the POS problem described above.

Only if you have trusted the right person to give you the correct genesis block.

In the case of bitcoin, I don't believe anyone could mine their own genesis block and build a chain even remotely approaching the length of the "standard" bitcoin chain. In fact, this applies more generally: after a blockchain has been in existence even a short time, it becomes infeasible for anyone to replace it entirely; at best they can hope to produce a fork from a fairly recent point as anything else would be a DRASTICALLY shorter chain.

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