Decimating is probably too strong of a term.
Elk numbers in Idaho are near where they were upon wolf introduction. A wolf pack hunts typically by separating one animal from a group - they're not often getting the strongest and best of the bunch in their kills but rather the weak and sick. Cattle such as sheep and cows die in far greater numbers due to disease than confirmed wolf kills. The actual percentage of deaths of cattle from confirmed kills is ridiculously small. Granted, there are likely a good deal more actual kills than confirmed, but the overall number is likely still extremely small as a percentage of the overall cattle population, and is still dwarfed by diseases.
One can look at the overall health of the grazing animals such as muleys and elk when their apex predators had been removed, vs now. At some point if left alone, nature will reach a balancing point - wolf numbers in Yellowstone have been roughly stagnant as have elk for a number of years now. When humans intervene, the end result is usually a less healthy overall environment - particularly when you factor in the overgrazing and the detrimental effects that has on the landscape.
150 - the number Idaho would like the population to be - is the number originally slated as the number the state could support, so the number is not arbitrary. And this bill doesn't open up to mere hunting by contractors paid for the state. Trapping is included, as is baiting. Night-vision equipment will be allowed as will shooting from helicopter. It's not at all what most hunters would consider "hunting." It's more along the lines of killing for killing's sake. If the number of wolves drops below 150, the management of the animals will be taken over by the federal government again.
The more concerning thing to many that oppose the bill is that it's been created by politicians as opposed to scientists, who are the ones that are supposed to be managing the animals while being far closer to the actual numbers of what the state can support naturally without drastically impacting the ranchers. Although I'm a Coloradoan whose done a fair share of research on wolves, I disagree with opening up for a vote to the public - the decision to re-introduce the animal. It should be left to the scientists who know the impacts and the benefits the best.