Thanks for that! I only heard Em's answer, not the question - and frankly, knowing the question makes a huge difference since it explains her answer.

This subject has come up recently on Tumblr, and the consensus seems to be that what Em said was true, that once Rumple blocked the kiss in SD, that was it - he'd made his choice. Some people suggested that the Darkness made adjustments so that "it" couldn't be destroyed that way again - and as one person mentioned, Emma "doubling down" on the Darkness (making Dark Hook) meant that the curse could only be broken by a sacrifice.