Without immutability, the blockchain will eventually lose all of its differentiating features, and have no value proposition.
But I think the parent comment is wrong about Ethereum being 'less immutable'. Either a blockchain is immutable, or it isn't. A year ago, for one moment, Ethereum was mutable. But that was a unique situation that Ethereum will never find itself in again:
It will never be that small of a community, and have that little happening on the blockchain. It will never be that young. Ethereum's leaders will never fail to that degree to properly warn people about smart contract security.
The Ethereum community will never be that inexperienced with smart contracts, both in terms of their security properties and their economic properties (the DAO was the very first smart contract of its kind, and had a goal of attracting $500,000 worth of ETH, and instead $150 million worth was deposited). Now that the community has gotten a lesson in how smart contracts work, there will be much less sympathy for anyone who loses ETH they deposit into one. It's buyer beware at this point.
So the likelihood of another hard fork happening to reverse a hack of a third party Dapp is extremely low in my opinion. Ethereum is for all intents and purposes immutable right now.
Can you do this without centralization?
>Without immutability, the blockchain will eventually lose all of its differentiating features, and have no value proposition.
Right my point is though that for most people immutability ISN'T actually a value proposition. It's a risk.
No, this is a service of centralized parties, which are free to use the blockchain. My point is that an immutable blockchain can have both mutable and immutable services, while a mutable blockchain can only have mutable services. The immutable blockchain therefore has a major competitive advantage over the mutable one.
With respect to immutability, it is important for some critical applications, like money in countries that restrict economic rights. If you live in Venezuela for example, you will not want your wealth in the form of a digital currency that's stored on the servers of a company in another country, because that server could eventually be shutdown, or have its ledger altered by the trusted third party running it. Your government is a powerful organization after all, and could lobby the government of the country where the server is held to force the company to restrict people from your country from using its banking services. Or that country may try to punish your country's government, by sanctioning all Venezuelan nationals, which again would lead to you losing access to the ledger.
You want you wealth in the form of a digital currency sitting in a decentralized network that maintains an immutable ledger.
If you live in a country like Venezuela couldn't they just shut down the internet and make your savings worthless overnight?
Regarding Venezuela, there may be situations where the government is willing to shut down your bank account or prohibit meatspace financial institutions from letting you as the client use other currencies, but isn't willing to go as far as shutting down the internet. So in that scenario, an immutable ledger would have value.
Also while the Venezuelan government may be able to prevent you from accessing the rest of the global internet, it can't shut the global internet down. So as soon as you leave Venezuela, you can regain access to your savings.
What about if someone steals your savings? How much do you value immutability then?
And FDIC style insurance does not work with a fixed currency, so you don't want to use these banks.