This is kind of a challenge because it comes down to what type of person an individual is. On one hand, if one reads Scrooge's transformation with a sense of positive optimism, then the transformation is valid. It is valid because Scrooge has seen the error of his ways. Dickens operates under the assumption that once an individual has been made aware of their ignorance, change is forthcoming and entirely possible. In this light, it is plausible to see Scrooge change as a result of his visits. If one is perceiving consciousness in a skeptical or pessimistic light, then the belief that change is possible once someone recognizes their ignorance might not be entirely valid, for ignorance alone does not cause the evil that humans do. In this vein, one is not going to buy the transformation, seeing it as more sentimental and maudlin than anything else. For this individual, Scrooge is not going to radically change the manner in which economic power is distributed. He still is going to be the dominant economic force and Christmas or not, little will change that. Certainly, Dickens believes in the authenticity of the transformation and it is evident in the text that he is visibly and emotionally shaken from his visit from the ghosts. If one accepts this as the basis for change, then the optimism in Dickens' work is evident. If one is more inclined to see the cynical approach in human motivations, then I think that there is probably going to be a greater amount of doubt regarding the plausibility of Scrooge's transformation.
Dickens was a great writer. He could make Scrooge's character transformation plausible while the reader was reading the story. In retrospect, however, the transformation seems destined to be only temporary because, for one thing, Scrooge is just too old to change very radically. Dickens cared deeply about the sufferings of the poor and underprivileged, but he had no practical solutions to offer. His story "A Christmas Carol" is typical of his views about improving the terrible conditions he saw all around him. Everybody ought to follow the teachings of Jesus in the New Testament. They should be kind to one another and love one another. At his best, Dickens helped to make people more aware of injustice and misery and then support practical legislation, such as child labor laws, that would results in real improvements in the conditions he exposed and deplored. "A Christmas Carol" is a "feel-good" Christmas story. Perhaps it would have been artistically improper for Dickens to show Scrooge changing too much or doing too much.