I mean, this IS television we're talking about, even if it's HBO which isn't quite as trigger-happy as the likes of Fox. Plenty of TV shows never make it past their first or second season - including the last show based on a major fantasy series, Legend of the Seeker. This will be rather different and a more ambitious project, I imagine they won't drop it too easily, but still chances are significant that somehow it won't get to season five or six, so the problem might not even arise.