From what I've read, they were definitely in terminal decline. Other producers were engaged in a "race to the bottom" that the Empire could not or would not match.
What Emancipation did was kill the island sugar economies quickly, sort of like being shot through the head versus wasting away from cancer. Because the "death" was so rapid, there was little chance for other economic activities to pick up the slack. Plantation owners who were watching their profits dwindle each year, for example, might have had time to experiment with other cash crops. As it was, the labor force immediately bugged out and - very understandably - worked for the former slavers just enough to feed themselves.
Economic consequences aside, I would have shot slavery through the head too. It couldn't be ended fast enough and the consequences be damned.
I know. I was just pointing out that from what I've heard, the sugar economy was not strong and vigorous before emancipation killed it. Like you say, it was already in deep decline. But I see that you know that.
And I don't think you need to argue with anyone here that slavery should have been ended as soon as possible.