So they can say some people can still use it one-handed.
Of those who've tried similar phones: some will like it as they don't need their other hand, others won't and will need their other hand anyway. And some will dislike accidentally pressing it because it's always under their fingers. As ever, you can't please them all.
As for the rumors, it's less double-down and more an express train.
I can use my 5 Inch Nexus 5 one handed most of the time. If the power button was on the top, there would be considerably more hand movement in order to turn it on/off. Also, I've never accidentally turned it off, and I've had it for 5 months now.
I can see why some people with smaller hands wouldn't like a 5.5 inch iPhone, but I think people underestimate how far their hands can stretch.