No, the population wont decline, because then they go after cattle. There is a hell of a lot more cattle than elk so they have no reason to go somewhere else.See dennishoddy's post above about the effects of reintroduction, particularly the bit about the restoration of habitat. With abundant food, species' populations grow, to the detriment of the food source. That's what happened with the elk and the local flora, and it's what's happening with the wolves upon those same elk. When the elk are reduced, the wolf population will also decline. Then, with reduced predation pressure, the elk population will rise again, leading to an increase in wolves, and the cycle repeats. We even have a mathematical model for it: its called the Lotka–Volterra equations, sometimes called the predator-prey equations.
I'm still not seeing where restoring a balance to an ecosystem that we deliberately unbalanced is a bad thing.