As you say, the appetites that both fanfic and fan art generally accomodate aren't particularly highbrow. I did fanfic for my consumption, but a sample art order went something like "Sailor Jupiter in her Sailor costume flashing pink panties and give her like a DDD-sized chest...er, make that EEE...actually, how big can you make her?" (At this point I thumb through my portfolio for the image that my wife used to call "beach balls girl.") "Awesome!! Just like her, only bigger!"
Yeah, fan art can be pretty fanservicey... I think I’ve seen some of those Sailor Jupiter pictures...
The thing about fanfic is you can attract a bigger audience than someone writing equally crappy original fiction. Which is a big motivator. But anyone who doesn’t get past that stage will never get any better. I feel sorry for the woman who wrote this 50 Shades book. She’s made lots of money, but she’ll never be a good writer.