Because he would still be their best attacking player at 5/8. He is suited to 5/8. The more ball you give this guy the better and who said he should stay on one side of the field? Long term, he should be all over the place. The best don't sit and wait for the ball. As for the extra tackling he will do, he has already shown that he can handle that no probs. So many lesser players then him are playing 5/8, so should it stop him from playing there?
He will be a 5/8 and probably the best in the world once he masters it after a year or two ala Lockyer.