Helix, despite your scorn, I would ask you to please read carefully, which I don't think you have up to this point.
You may find it difficult to believe that lions were vegetarians less than 6000 years ago, but I don't. And I don't as someone who has studied biology and taught it.
First of all, it is important to remind both of us that we don't KNOW any of this -- on either side. I used to be on your 'side', thinking creationists, and in particular, young earth creationists, were nuts, ignorant, blind...choose your adjective. I'm trying to say that I know what you think in this area. I spent five years reading due to some student questioning in my late twenties and early thirties. It was due to data, particularly in the area of population genetics, which convinced me that not only could evolution not happen, but that life, and possible the earth and universe itself, were quite young.
So, play the game with me for a moment. Let's look at it as if that were true regarding the points in your post.
First, we know that functions can be lost. Cave fish have lost their eyes/sight. This has nothing to do with getting eyes in the first place, but with the fact that when something is not used, there is no pressure genetically to select it. And if some other trait is important and more or less accidently involves the loss of another function -- that is something we see in genetics and purposeful breeding. I'm sure you know of the various weaknesses inherent in certain breeds of dogs which have been inbred extensively. Deafness in the Dalmation, hip dysplasia in the German Shepherd, etc. There are trade-offs in traits genetically. This, by the way, does not stop a berry from being a berry or a lion a lion.
So here we have the lion, a confirmed carnivore.
Or is he? Not totally. The contents of the prey's stomach are actually important to the diet of the lion or any other carnivore. That's the first thing. Here is a web page about feeding the large cats in captivity:
http://www.2ndchance.info/bigcatdiet.htm
It does not mention stomach contents, but it does mention that there must be additives to a plain chunk meat diet. I don't have the time to research it tonight, but there are cases of big cats preferring vegetarian diets. There is the story of "Little Tyke", but I honestly don't know how true it is: http://www.essene.com/HumaneReligion/LionAndLamb.htm
What does need to be recognized, though, is that all living things need certain nutrients, regardless of where they get them. We do know there are some plants high in varous proteins, such as soy. We know that some plants eaten in combination provide most the essential proteins and amino acids found in meat -- such as rice and beans together.
We also know that there are some plant products today which need a great deal of strength and/or cunning to get to, such as coconuts. We also know nuts are high in protein and have some amino acids.
I think you can see where I am heading with this. If this is a young creation, which I believe it is, and if Noah's flood was indeed a world catastrophe, which data tells us it was, it is not far-fetched to consider that there were many, many species of plants available before the flood which did not make it through.
If any of these species had large, hard nuts which needed powerful jaws to crack, and whose nuts were jammed with the proteins and amino acids needed for todays carnivores, then we have the answer to the big cats. I think also of the saber-tooth tiger -- what were those extraordinarily large teeth for? Digging, like the walrus?
In other words, when I put together what we do know: traits can be lost (such as the ability to consume grass and leaves?), plants can be sources of protein and amino acids, etc., I am not relying on pure imagination to think that it was indeed possible for todays carnivores to have been yesterday's herbivores, PROVIDING the proper plant life was available to them.
It is certainly reasonable to assume that the big cats were ALWAYS highly dependent on a high protein diet -- their intestines are indeed apparently designed for that. But what is up in the air is whether or not the source of that protein was always the flesh of other animals.