Cancel the report will be a good feature to have but not sure how many members will actually use it. I never go back to track the reported post.
I would have used it quite a few times in the past few years, especially when I was doing a lot of shitpost reporting all at once. There were a number of times I went back to a member's post history and ended up double-reporting a post by accident. It would have been nice to have been able to cancel those reports, and I think it would be a good function to have. The mods would be benefited by it as well, probably even more so than reporters, since if posts are erroneously reported they have to figure that out themselves and waste their time doing it.
I don't really care much about my accuracy, since I'm not shooting to be one of the top reporters or the most accurate--but if this wouldn't be too much of a pain in the ass to implement, I think it would be a good function to have. Personally I hate making a mistake when reporting a post just because I feel like an idiot.
How is canceling something pointless?
It's not at all pointless IMO.