Imagine when (if?) Winds is completed/released and Martin goes on an admittedly well deserved book tour. But then the tour lasts too long, and he comes home and is behind on the 5-6 projects he's working on. Meanwhile he's accepting other projects, other speaking engagements, etc. All of a sudden nearly two years have passed since Winds' release and he has barely written anything. Meanwhile the show has ended, and his story has effectively been spoiled. Thrones Mania is declining but he's trying to revive it by being involved in the spin off production. Another year passes. And the more Martin measures his progress as well as the plot, the more he realizes an 8th book would make things easier.
Good luck.