There are likely a lot of reasons GRRM won't let the show finish first. Primary among them is that he would have to collaborate on how it works out. He wrote for TV prior to his novels, and he became a novelist to tell his stories his way. Why would he resort to collaboration for the climax of his seminal work?
But, I doubt money would have any bearing. Maybe the publishers wouldn't want sales hurt, but GRRM is making a ton of money from the show and his book deals regardless of timing.
I think you nailed it. Finishing the TV series first would lock Martin into decisions that he wouldn't necessarily be settled on. From what I've read, he revises each book extensively before showing anybody else.
As long as the time between A Dance With Dragons
and The Winds of Winter
isn't as long as the time between A Feast for Crows
and A Dance With Dragons
, it should be out by the time the show catches up to it. Even if they've made it through the end of Book 5 by the end of Season 5, that gives him two years to get the next book out. The real question mark is whether Book 7 will be out in time.
If it's not, I think the best way to handle it is to simply put the show on hiatus until it comes out. Lets say that Book 6 comes out in 2015 and that the content of Book 5 has been wrapped up in the show by the end of Season 5, also in 2015. They adapt Book 6 over Seasons 6 and 7, and now they're caught up in 2017. Assuming their character survive that far, at this point, Maisie Williams will be 20 and Isaac Hempstead-Wright will be 18. The longest gap between books so far has been six years. By 2017, under this scenario it'll have been two years since Book 6 came out. So even if the last book took just as long, it'd be a maximum four year wait. And Williams and Hempstead-Wright will already be pretty much done growing. There was a three year gap between the end of Season 5 and the end of Season 6 for The Sopranos
, so it's not without precedent for HBO.