Most succesful projects have followed this similar path.
Immutable?? ETC said good bye to immutability with this attack so this word has no connection to ETC any longer.
51% attack is not about immutabilty. ETC stands for immutability at the smart contract level. Of course, ETC devs should apply greater efforts to prevent POW attacks.. change also, turn to hybrid POW/POS or smth. unique move. They are smart people.
See most of the potential coins are migrating POS model in order to generate gas for their transaction but we don't know how far this POW model will work. We need to wait and see the price of ETC will be in the next year.