But again, you're talking about two separate things there. Bitcoin is 100% immutable UNLESS it's changed. Right now, the feature of Bitcoin is immutability. So we can trust that it will remain immutable until it isn't. If it isn't, that's because it was attacked or because the devs forked it to be mutable. Since the devs have not forked for MtGox or Bitfinex, I see no reason why they would do this.

Therefore, what you've described is an attack on a feature of Bitcoin. Mining pools getting together to roll back blocks would be an attack on the blockchain. That doesn't make it any less immutable; that means it has been attacked.

Immutability is a feature; changing that through miner collusion is an attack.