To be perfectly honest... I don't see the HBO series going the entire distance anyway. But even if it does, I don't think they're going to stick to a "one book/one season" format the entire way. I'm pretty sure they've already discussed making
two seasons out of
A Storm of Swords. And of course,
Feast for Crows/A Dance with Dragons will have to have a completely different treatment for television. A TV audience won't tolerate the popular characters disappearing for an entire season (ratings would plummet), so I expect the events of those two books will be interleaved (for a total of 2 - 2.5 seasons).
Plus, the way I understand it, HBO wouldn't
have to wait... they've been given a general outline of the way things might wrap up—in the event something happens to George or the book series. And for all legal intents and purposes, the TV show is autonomous. They can deviate if need be. All they're obligated to do is offer George the opportunity to write one episode per season.
That more than anything is what I'm hoping will inspire him to stay ahead of the TV series: vanity.
(I don't mean that disparagingly toward Mr. Martin; I would be vain about my own creation as well)
I don't see the book series wrapping up in seven, regardless.