They can't hold all those actors on contract forever. It's been 7 years already....the kids are growing, the last books aren't out yet and god forbid anything happen..........you can't expect them to wait indefinitely for the books to come out and just prolong the series and eventually write themselves into a LOST..
When did I say they should wait for the books? D&D knew the actors were only going to stay for so long. This isn't new information. Heck, there are 3 more episodes (73 vs 70) than they expected to do. If they don't have enough time, that's all on them. Why absolve them of their own decisions?