The main rationale is that after ETC intentionally breaks several smart contracts on-chain, it has become essentially the same thing as Ethereum, even in terms of immutability. Without the distinguishing factor of immutability, supporting ETC becomes a breach of majority consensus with no good reasons, and thus against the ethics and principles of maximizing consensus and user choices.
A different roadmap (such as staying PoW) is not a good reason to cause the split *right now*. The split should happen at the last possible point when Ethereum finally moves to PoS.
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