Oh, this is an interesting meta question!
Generally we are very careful not to change a question once an answer is posted, but I agree that what you are proposing doesn't really detract from or negatively affect the answer, other than preventing it from becoming "the" (accepted) answer.
Generally list questions1 are discouraged in Stack Exchange; questions requiring a long list of things as answers, as the answering can become unwieldly.
However there are plenty of well received exceptions to this, and it turns out that SE encourages questions that have many answer posts; multiple answers per question is even used as a basis for site graduation!.
So in this particular case for this particular question and topic I personally feel it will be fine to adjust your question to something like
What are the different approaches that can be used to determinee/estimate/calculate X?
or similar. You can leave a note under the current answer letting the author know of the adjustment (they don't get a notification) and explain that you appreciate their answer and it made you realize xyz and so have adjusted the question.
1For more background and discussion and thinking about "list questions" in the main SE meta: