Unfortunately, Ethereum did infact mutate its chain. Bitcoin never mutated the chain, but once a fork was done to abandon the original chain from a few blocks back, and start mining a new chain from there, thus invalidating some transactions. However Ethereum did something which is essentially a crypto-blasphemy. They reversed a single transaction without a valid signature. Ethereum-Classic is the original chain that still contains that transaction.
The fact that they created a new chain diminishes one of the main characteristics of a blockchain. That it can't be altered.
This has always been my biggest sticking point with cryptocurrencies. It doesn't take into account that sometimes you need to fix a mistake.
Cryptos also have a usability problem. If you forget your login credentials you lose access to your account. If you forgot your credentials at a regular bank, they have multiple avenues for figuring out if you are who you say you are. Eg post etc.
Bitcoin did not break any "technicality". Technically speaking, the chain with most work is to be considered valid. What they did back then was, a bunch of miners got together and created an alternate chain with more work. So it's still technically the correct/longest chain. On the other hand, Ethereum just plain broke the sanctity by basically creating a chain that contains a transaction without a valid signature.
Chains are amended, not mutated. Consensus about what new blocks to accept to the chain can change at any time. For example the chain following the original Ethereum rules is now called Ethereum Classic and its market cap is less than 0.5% of the new consensus.