"licensing usually cheapens the original creation by saturating a market with characters until readers are bored of seeing them" makes the most sense to me out of this. When I think of Shrek, I don't even think of the movies, I think of all the random stuff like Burger King that licensed it.
Also, now that I've read this, I'm kinda sad about the bootleg peeing Calvin truck decal.
> Owing to spite or just a foul mood, have you ever peeled one of those stupid Calvin stickers off of a pickup truck?
I figure that, long after the strip is forgotten, those decals are my ticket to immortality.
(from the Mentalfloss article linked a few comments down)