I think I get the point of your questions. I think that you are interested in talking about the physics or science or philosophy of killing big game animals.
I understand the point about energy retained by a passed through bullet being energy NOT delivered on the target. If 1500 ft lbs of energy are retained by the bullet just prior to impact and the bullet passes through with a retained velocity of 200 ft/s and 80% mass, then something less than 1500 ft lbs of energy were delivered on the elk -- just wildly guessing, say 1300 ft lbs of energy is delivered on the elk and 200 ft lbs of energy is retained by the exiting bullet. Why is this better than the case where the bullet does not exit and hence all 1500 ft lbs of energy is delivered on the elk?
One of the posters, I think, suggested that the value of having a bullet exit is more related to making tracking easier than it is related to killing the elk. So this can be thought to be the prime virtue of the bullet passing through.
As another poster mentions, you want the bullet to retain mass and not break apart, because the bullet is more likely in this case to retain energy and penetrate deeply. It is by penetrating deeply that the best opportunity to maximize damage to the vitals is realized. As an extra bonus, if you bullet mushrooms, you cut a broader channel and produce more damage.
You could say that energy doesn't kill, it is deep penetration that kills. And a broader projectile produces more damage than a smaller projectile. For instance, I doubt that arrows kill elk because of the energy they have (that is, 1200 ft lbs of energy are not thought to be necessary to kill an elk with an arrow). I imagine they kill because of the cutting effect of the broadhead. The broadhead is able to penetrate deeply because it cuts through tissue. A bullet penetrates in a different manner -- it doesn't slice through.
Put all this together and it means you want a bullet which mushrooms but doesn't break apart. If it does this and it also penetrates all the way through -- then it seems to me you get maximal damage to vitals plus you get a good blood trail if you need to track.