how would anything ever be immutable if people can reassign the symbol/pointer/name?
the DAO hack happened, immutably, no one disputes it. the hashes and blocks and transactions are well-known. so there was a "schism", that explicitly validates the fact that without this large-scale cooperation, without the redefinition of what Ethereum is, it would be still be what is on that other branch. these both provide evidence for the immutably and decentralization.
The version of Ethereum after the hack became known as Ethereum Classic. The Ethereum foundation decided to go with a fork of the chain prior to the hack, and pretty much all the devs and the community followed. The value of Ethereum is entirely derived from what people are willing to pay for it, and community is a big part of that. The version of Ethereum which underwent the attack didn't cease to exist, and people can still use it; it's just called "Ethereum classic" now, whereas people who want to use the version of the chain that didn't suffer from the hack can use that version (generally understood to be "Ethereum".
The fact that there are far fewer users of Ethereum Classic (and the market cap is significantly lower) is a testament to how much people care about the community which chose to follow a different history of the Ethereum network.
Small nitpick. In both chains the attack happened.
But in one chain the whole community decided to disown the attacker by injecting hard coded transactions that would send the Ethers back to their original owners.
It wasn't a rollback in much the same way that UPDATEing a row in an MVCC database doesn't actually overwrite that row, it just creates a new version of it that becomes the version that people tend to care about from that point on.
the DAO hack happened, immutably, no one disputes it. the hashes and blocks and transactions are well-known. so there was a "schism", that explicitly validates the fact that without this large-scale cooperation, without the redefinition of what Ethereum is, it would be still be what is on that other branch. these both provide evidence for the immutably and decentralization.