I don't think xkcd is a good example of practice making perfect. Many former xkcd fans think that the average xkcd comic became noticeable less funny somewhere in the low to mid hundreds. Part of it was the novelty being gone but part, I'd say, really had to do with the quality of the comic's writing.

The author's growing experience and confidence allowed him to pull off ambitious and genuinely imperative things like "Time", which improved the mean comic, but the median never really recovered.

Yes but we're not talking about how funny the comic is just the how good the illustration has become.

Instead of "genuinely imperative" I meant to write "genuinely impressive".