I will always answer all comments on my posts, fic or not. Sometimes with a few days delay but I will do it. And I agree with your point whole-heartedly. In an unlikely scenario that person would both get 10 pages of comments AND be super-duper busy at the same time, it takes 1 minute to edit the post to leave a general "thank you everyone, will get back to you soon" message at the bottom of the page or something.