It's not 51% of the coins, it's 51% of the computing power on the network. Both of which are virtually impossible in the case of Bitcoin, though not entirely impossible. I just wouldn't consider a 51% attack even remotely a threat to the network compared to something like government crackdown

Immutable so long as no one party or group owns more than half of the coins on a given blockchain... then the ledger is whatever they say it is and it propagates down because they can manufacture their own "consensus".

No, the community controls the consensus through their nodes. A 51% attack only allows the attacker to perform:

1. A double spend attack - sending a transaction, receiving the goods and then reorganizing the chain to undo the transaction.
2. Censoring transactions.

In the event of a 51% attack the community can fork the chain - change the consensus and implement preventive measures like changing the mining algorithm, changing to PoW/PoS, banning all of the attackers coins, implementing a finality layer or a checkpoiting system etc.

You are making my point. Blockchain is not crypto. Blockchain can be useful in private, internal use cases (like a transaction ledger for bank branches) where trust is largely implicit.