My question was actually provoked by the treatment of this question but let me make up a fictitious example to sharpen and clarify the issues.
Suppose someone posts a question asking for the first story in which Earth is invaded from outer space. Somebody posts an answer with a 1995 story arout an invasion from Rigel, and then somebody else posts an answer with an 1898 story about an invasion from Mars, which is accepted.
Sometime later someone posts a question asking for the first story in which Earth is invaded by beings from Rigel. The new question is immediately closed as a duplicate of the old question, since it is "answered" by the 1995 story.
Subsequently, more invasion-from-Rigel stories are found, earlier than 1995 but later than 1898. These are better answers to the newer question, but they can't be posted anywhere, since the newer question is closed, and the older question already has a better answer.
Of course the newer question could be reopened, but should it have been closed in the first place? By what right did we assume that an answer to a different question was the correct (i.e. the earliest) answer to the new question? And didn't closing the question lower the chances of a better answer being found?