I had a similar epiphany recently while reading a very popular AU fic (which I usually don't read, and I have no idea why I'm reading this one), which is actually well-written and meticulously plotted. I stopped for a minute and thought, "Why isn't there a book like this? Because I would pay for it."
The only reason this 50 Shades bullshit really bothers me is that it's BAD fanfiction. There is a lot of good fic out there. This shouldn't be the bit that gets a multi-million-dollar publishing/film deal.