It is inevitable to lose players as time passes on, particularly as other big name fighters come out. Heck, I just have to look at my friend list now, and the majority of them are playing far more SF4 than SC4. But... there is an increasing number of them coming back to SC4.
Broken Destiny is sure to bring back interest to the game among the less hardcore. Though if it remains PSP-only, then, it won't help the main community too much. But, still, its bound to bring in new players to the game. And if its training missions thing actually proves decent, then we might actually see more players who know what they are doing out there (though that's more likely only if SC5 eventually includes such a feature). I'm sure there will be plenty of information at E3 for us here.
As for Nats/Evo... there will always be more tournaments. Heck, have another nationals tournament, there doesn't have to be just one.
But, the hard core SC4 players will stay.
You also can't judge things by the amount of posting going on, as the majority of things about the game have already been discovered and talked about. Or, the questions that people have are already answered somewhere (assuming people search...)