Ah, but how are you getting the data from the " some significant number of the theoretically possible audience" to the extent that you know it is significant? If your population approaches infinity, how will you know what a signficant N is?

Whereas you can survey the satisfaction of feedback where N = 1 (or slightly more than that with coauthored works, but it's rare that N > 5), so you can only ever know if feedback is good. You can never know if a fic is.

Also, I submit that authors are very easily pleased and if you point out a couple things you liked, you will make that author's day, thereby contributing more Goodness to the world. (Also, you are adding positive reinforcement to continue to get more fics, which you have defined as Good by giving positive feedback on it, thus contributing to your unbounded set's population)