What does Motion's article say about excellent sequels? The summary of the article on the index page of "Sunday Book Review" reads as follows: 'The best sequels enrich and extend the original story, but they do not treat it as an unfinished work that needs completion.' At first glance, I did not fully understand the meaning of the latter half of this summary. However, the sentences of the article related to this part of the summary were more understandable, though they repeatedly used the word "they" to mean different things: '… best sequels do not intend to "finish" whatever stories they [best sequels] tackle because they [the authors of best sequels] think they [the original stories] are somehow incomplete. They [best sequels] are instead based on the assumption that no story is ever finished — and present themselves as a chunk of life, not as complete and rounded histories.' (Words in [ ] are my interpretation.)
I found Zoku-Meian quite an enjoyable sequel. Did it not intend to complete Sōseki's original work? I think that it did. It had a final scene, in which the main character felt something like a revelation (if my memory was correct). This scene gave me an impression of being a little out of place, because it was not like the one written by Sōseki but by some other author, for example, Saneatsu Mushanokōji, in spite of Mizumura's effort to write in a style as close as Sōseki's (though, according to a review, found on the Internet, of Zoku-Meian, Mizumura wrote in the afterwords of the book that such had not been her main goal).
On the other hand, the style of Mizumura's sentences in the sequel was much like those of Sōseki's original, which had been written more than seventy years before when cultural environment and the use of Japanese words were rather different from the present day. What interested me most seems to have been this aspect of the sequel by Mizumura. Thus, masterful imitation of the literary style of the original author seems to me to be another significant, independent factor to make a sequel excellent.