Good topic. Here is my take on your question.
First, we should not try to kill an animal at distances that the bullet will not carry lethal energy. Not worrying about accuracy, which is also a huge factor in this equation, purely the ability of the bullet to penetrate and shed energy in the critter that will cause death or incapacitation quickly.
Second, we should not try shots that are beyond the ability of the shooter and the rifle/scope/bullet system to consistently and confidently hit a vital area.
I guess my take is that the answer is a blend of the shooter's ability and the bullet's performance once launched. I cannot get excited about paper ballistics and energy. As hunters we should ensure that our bullet gets to a vital zone with adequate energy and penetrating ability to kill - or we should not shoot or should simply try to get closer.
I just returned from a caribou hunt. For the first time I took a .308 Win. instead of a magnum. Fact is I was so confident in the .308 rifle that my "lethal range" was significantly farther than on previous trips during which I hunted with .300 magnums.
Had I prepared similarly with a magnum, the range might have been a bit longer, not sure as I was prepared to shoot out to 700 if the wind allowed and the critter was standing.
Does the .308 have enough energy to kill at 700 yards. I don't care what the energy tables say, I know that it does. Particularly with good bullets.