No it's not realistic at all.
Animals today eat just exactly what they are adapted (or "designed" for you creationists) to eat.
Cats for example cannot internally synthesize the essential amino acid, taurine and must therefore obtain it directly through their diet. The problem here is that taurine is not found in plant tissue, it is only found in meat. Ergo, cats are not naturally capable of being vegetarians.
If you carefully read JW publications, you may get the impression that some writers realize what a load of nonsense the idea of an all vegetarian ecology is. One of my best friends, a JW veternarian, actually wrote them a long letter after a series of particularly inane articles came out in the Awake! in the early 1980's, so this has been explained to them at great length.
However the JW's have never renounced this idea. It is still "official doctrine" as this picture from a recent Watchtower shows:
Straw (The biologically inactive, dried out stems of grass) is not a suitable food even for cattle and horses. Yet JW writers seem to feel that the lion is somehow capable of eating it.