If the readers already know how the long epic will end, and those who just keep reading because they've already invested so much time and they just want to know how it ends, no longer have to do so, that means Martin will have to ensure his book is worth it on its own merits, not just the coattails of the previous books.

In practice, I'm inclined to say that it means he will probably lose much of the huge numbers of extra readers he gained through the show, the bandwagon folks who figure that geeky fantasy books become cool enough to be seen reading once they've inspired a TV show (such as some newspaper book reviewers that I could name, whom you'd expect to have more serious standards, but I digress). Go back to basics, as it were, to his old fanbase, which was sizeable enough within the fantasy genre. Because I can't see too many of those giving up on the books just because the TV show spoiled some of the ending. They may just tune out because they've outgrown the series, but that's something else that could happen just as well without the TV series.