It's not that
GW is not realizing them to their full potential, its just that every race and faction pretty much has their "I Win" button that could be brought about due to luck/victory and the like. Well, besides the Tau.
You realize that all the codexi (at least the older ones) are written and from the perspective of the Imperium of Man, thus are colored by their prejudices; and, since the imperium is superstitious, everything that is a threat to their empire is viewed as OMG THEY WILL KILL US ALL! This is never more apparent then in the Tyranid Codex; they are a new, frightening threat that has come at the absolute worst time for the imperium to handle.
Even if the Tyranids devoured everything in the galaxy, there will be at least three races that live:
1) The Orks- in the fluff, Imperial Scientists sent out probes into deep space, beyond the edge of the galaxy. Supposedly, the only thing it would consistently pick up beyond the rim of the
galaxy is Ork chatter. The Orks, much like the Tyranids, have spread out far beyond the galaxy that has give them birth. The only thing they are lacking is a pressing need to consume
everything; since all they need is a good fight, they will never become unified unless the opponent is daunting enough . Who knows, maybe eventually the orks find that "galaxy far, far
away," and return with a massive waaagh of trillions tooling around in looted Deff Stars!
Also, remember Orks are currently the most populous race in the galaxy, so who knows
what damage could be wrought from a galactic Waaagh!
2) The Necrons: The Tyranids avoid them like the plague for two reasons, one for certain and one that is conjecture on my part. One: they are inorganic and the worlds they inhibit are
sterile, so there is no reason the nids want to engage them in warfare, since it can only cause a depletion of the hive's biomass. Second, what ever passes as the collective consciousness
of the hive-mind probably senses a sense of dread from the worlds necrons occupy, since it
is a warp-entity. Hell, Necrons would probably view the 'Nids as a great source of life to
harvest for their C'tan overlords.
3) Daemons: They will be radically altered with the destruction of the eldar and humanity, no doubt. Since the Hive-Mind is the tyranid's "reflection" in the warp, they probably won't be
getting any weird daemonic genestealers anytime soon, but will probably start to mimic most strongly the last race around; the Orks. We already know that Gork and Mork are some of
the most powerful warp entities, so maybe the sudden monopoly on the warp the Orks will suddenly hold will start to have bizarre effects on Orks. Before that happens though, don't
count out the human and eldar chaos gods, especially Tzennech and Nurgle. I don't doubt the Great Mutater has a few tricks up his sleeve to counteract the tyranids, and maybe we will
start to see some diseases cooked up to target tyranids especially.
Also, remember that when the eldar are wiped out, it will cause their god to reawaken and cause all sorts of
, and if the emperor awakes, the galaxy is going to simultaneously get a lot more daemonic and whatever remains of humanity will have a central figure to reunite under. I just feel sorry for the Tau; they are one bad day away from being wiped out utterly.
Anyway, until
GW advances the plotline more then 200 years at a time (
) we'll never know what is in store. Honestly, people are probably going to think that the races they like the most are the ultimate badasses, and there is mounds of fluff to support any one of them gaining dominance (tau non-withstanding). Tyranids are a deadly threat, but their biggest advantage is that the entire galaxy is too busy fighting each other to properly bring full force to bear on a common foe.
Primary Army-- 1903 Points
Smurfs 
. No I don't believe they are the one unstoppable force, take a look at the Necrons, and once the Emperor is eliminated (keeping our fingers crossed) the Chaos Daemons.