Eh, if he lost interest that early, he could have greatly cut back on the complexity of the series. Greyjoy and Martell drama, while great at times, could easily have been minimized or cut entirely in favor of keeping the action on the Lannisters, Starks, Baratheons, the Wall, and Dany. That's still plenty of interwoven sotrylines.
When I was reading the books, I expected the end of Storm of Swords to be the point of the books with the most storylines going at once, and with a few crucial deaths, the stories would begin to converge and simplify. Instead, GRRM added a bunch of new characters: some good, some bad, but all of lesser importance than the ones we've been following since the beginning.