I'm on my phone right now so I can't pull up the photos, but both Schultz's and Breathed's art changed dramatically during their tenure. Breathed's over the course of 9 years, even.
The difference is that Schulz and Breathed's styles evolved into something
good.
Yes, Snoopy changed a lot from his original version, but he stabalized in the 1970s, and it's a really good design that was settled on.
Even the radical shift to a more cartoony style was with a purpose. Snoopy's
personality changed along with that look. Snoopy went from being a fairly realistic dog, to the almost human character that he is now. Snoopy's look was
refined with an eye to consistent characterization, so that he'd be a recognizable icon. The same thing happened for Garfield, and Hobbes, and pretty much any character of note.
Penny Arcade's art, as best I can tell, is just Gabe experimenting with different art styles. It doesn't seem to be connected to characterization, or with an eye to be consistent. It's just him playing with art. Which isn't a bad thing, in and of itself, but in this case it's resulted in a style that I think is disconnected from the comic and the characters. Gabe is supposed to be a father, but other than being taller than his son, they look the same age. The art isn't serving the characterization.[DOUBLEPOST=1435794969,1435794891][/DOUBLEPOST]
Gabe gets a lot of shit for his art, but he didn't start getting work outside his strip until after he became more experimental. Now he gets to do art for WoW cards, comic books, and other shit... ON TOP of his multi-million dollar media empire.
I think this might be a case of correlation not equalizing causation.