Why is no one seriously talking about HBO taking a year long hiatus from making GoT?

It seems, given the timelines involved, that GRRM will have book six done in time, but most likely not book seven. But, if he is to be believed, he will not be THAT far behind. So instead of these debates about how evil HBO will be when they finish the story off before he does and yada-yada, why aren't we talking about real ways to address the issue.

All of the child actors will be sufficiently grown up by the end of season 6 that an extra year won't make them look any more adult than they already would have (with the notable exception of actual newborns who are going to be recast as they age anyway).

Would there be any harmful consequences to a hiatus? Would it be realistic? Would it really give GRRM enough time? These are the questions I think we should be asking, not "What if HBO finishes it first and no one wants to read the book."