OP's argument is that this distributed consensus algorithm works only if there is already consensus about which algorithm to use in the first place! If you and

OP's argument is that this distributed consensus algorithm works only if there is already consensus about which algorithm to use in the first place! If you and I disagree about which block history is valid (for example, because my history includes blocks with a lower difficulty), then the assumptions are no longer valid and there's a hard fork. Also, note that a 51% attack can only exclude transactions or allow double spending - the ledger still can't be arbitrarily manipulated since transactions are signed by private keys.