If it solves the question, it is exactly the right answer, regardless how much time passed.
The problem is that people mark answers too fast as duplicates, so as time passes and tools develop, questions get closed as duplicates even when the question is slightly different or the answer won't work any longer.
> If it solves the question, it is exactly the right answer, regardless how much time passed.
But that's the crux of the matter - what exactly is this "question" entity?
I'm reminded of Heraclitus's "You cannot step into the same river twice" - if a question is an utterance made by an individual at a given point in time, then it might have "exactly the right answer" that they'd accept, but then it's extremely sensitive to those conditions, and even an identically worded question asked by the same individual a week afterwards might merit them to choose a different accepted answer, because the context for their question changed. For example, something like "What’s the proper way for me to deliver a signal to a different thread?" can have a myriad different contexts, and even if you give a wall of additional information and code, there probably would be something important omitted.
On the other side of this spectrum, we treat questions as a pointer to an underlying platonic idea of the question, which is exactly what StackOverflow say in their guidelines [0]:
> There are many ways to ask the same question, and a user might not be able to find the answer if they're asking it a different way.
Indeed, the closed-as-duplicate label uses the following text make it clear that they approach a question as an idea independent of the context in which it was asked:
> This question already has an answer
So if a question is independent of its questioner and can lead to other questioners being told that their utterance is a duplicate even a decade afterwards, then why should the original have any extra rights in deciding which one answer is the right one? Shouldn't this be owned by the community? At the very least, if a questioner writes a good question that is marked as a duplicate, then they should be given the same access to decide on the accepted answer to the merged question.
The problem is that people mark answers too fast as duplicates, so as time passes and tools develop, questions get closed as duplicates even when the question is slightly different or the answer won't work any longer.