I'm still torn on both sides with theDAO fiasco. I'd like to do a hard fork so I can get my coins back, as right now theDAO is pretty much dead moving forward. But does this cause a concern for immutability and would it destabilize Ethereum itself? Supposedly theDAO shouldn't be changed once its written into code. How does this affect the reputation of Ethereum if its immutable or not? Suggested best course of action?
By definition a hard fork erasing the DAO hack from the blockchain would mean that the blockchain was not immutable in this instance.
There are some persuasive arguments on both sides that Ethereum miners (voters) will have to balance:
The importance of an immutable blockchain versus the desire to refund innocent victims of the attack by using a hardfork to recover the stolen Ether.
Is "fairness" more important than an immutable blockchain? Will future currencies or DAPPs that care greatly about immutability think twice about building on Ethereum if the miners approve for a hard fork now? Will regulators use evidence of a hard fork today as support for pressuring developers/miners to hard fork again in the future?
If the price increase of Monero the day the Ethereum hard fork solution was presented is any indication, there is likely to be an exodus of some Ethereum users/developers to more immutable/fungible alternatives if a hard fork occurs. Many blockchain purists believe in immutability more than almost anything and will cite that MtGox hacking victims never asked for a hard fork to refund their Bitcoin losses.
The great majority of Ethereum users do not think it is fair that innocent DAO investors were robbed by a hacker and truly feel bad for the victims. The economic value of the hack truly was substantial (although perhaps less in % value of ETH market cap compared to the % loss of the MtGox hack compared to BTC market cap at the time).
Should the value of the loss dictate whether or not a hard fork is required? Should miners be worried about a slippery slope and requests for more hard forks in the future from victims of fraud, hacking, or regulators?
How will the miners vote? Right now there are more questions than answers.
I don't think this is the place for a political debate on whether we should hard fork, so I'll restrict myself to a narrow technical point.
It has always been the case that blockchains are not completely immutable, since >50% of miners can always change the rules with a hard fork. This is true on Bitcoin as well as Ethereum. If the miners decide not to hard fork in this case, they're still making a decision about the rules.
Blockchains are really about consensus, not absolute immutability. Even Satoshi's Bitcoin paper concludes that nodes "vote with their CPU power." We delegate decisions to code, but we can always decide what code to run. Bitcoin and Ethereum have both hard forked before, and Ethereum plans to hard fork on a regular basis as it advances the technology. Some of those forks will have economic consequences.
Despite that, blockchains are mostly immutable, since it's no small matter to get >50% of miners to agree to a change. It's controversial even in this case, with a theft of $50 million affecting thousands of people, where the money is unspent and can be returned to investors without having to roll back transactions.
So I don't think it violates a basic principle to hard fork. It's just a policy decision. And we don't have to have the same policy now, when the technology's brand new, that we'll have five years from now.
No one ever pretended the blockchain is immutable. The blockchain is driven by consensus. If forking is accepted by consensus and thus blockchain is modified in a way to lock ether in the dao and then provide a way to withdraw it, we are absolutely doing it the way Ethereum was designed to work. Consensus. This is called direct democracy.