It's all speculation really. People have been saying for a while now that in just a very short time it would be no longer worth it - yet it continues to be worth it. Which isn't to say it will be worth it for a long time, just that no one really knows or can even make an educated guess. That said, mining will always be borderline profitable at the least. While some people don't have to pay power bills, they are in the minority. So in some sense it will always be worth it (in the scenario that bitcoin remains used) depending on how patient you are.