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.

To me that recent one is an example of overdevelopment of the art.

I remember commenting on a different artist's rework of his old comic strip, about how the new strips looked too busy and that there was something beautiful and thematic about his earlier work. He replied that he also preferred his earlier work, but he just couldn't make himself 'stop that early' when drawing anymore.

Certainly in both cases there's a lot more skill in the later work, but it's interesting to note that overdevelopment is also a thing.