Well, let me amend things a bit. Okay, "Cash grab" may be a little overboard and harsh. I'm sure Winds of Winter would sell better than this if it were ever published.
However, I think this prequel is pretty damning evidence that Martin is bloody well BLOCKED on the main story. He probably has a definite ending in mind, but I believe the story has grown out of hand and he has no idea of how to get from Dance with Dragons to that ending. Many criticize the HBO show for having become pretty much standard fantasy fare since going beyond the books. And there very well may be some truth to that. But at least Benioff and Weiss are giving us a resolution to plot lines that have been going on since 1995 when the first book was published. I don't think Martin will ever do so.
I really can't see myself buying the new books once the HBO show wraps things up, MAYBE in paperback, but definitely not in hardcover.