IMO you have it right there......this isn't a mystery.
I'm only 34yrs old but remember "back in the day" when I was a kid that red fox prices were $60-70, Grays something like $40? Anyone who could set a trap was trapping back then. Farmers shot hawks on sight, pole traps were used etc.
The impact of predators on small game animals isHIGHLY underestimated IMO.
I have no way to back up these claims but I "feel" like predatory animals and birds are the majority reason for the decline of the Pheasant in PA.

I get a first hand view of it DAILY at my house and I live just outside of the center of a small town. We have a nesting pair of Red Tailed Hawks that staked claim here 3 yrs ago. I have watched them systematically wipe out every sign of life that wants to enjoy a little sunshine.[:-]
In the last month alone I watched them take 2 squirrels, the only rabbit I have seen in over a year that dared to come out in daylight hours, a muskrat, a groundhog , a black snake, and several mallard chicks. And this is just what I happened to see by sheer luck of looking out the window at the right time.

Not saying that we should be shooting hawks, and I enjoy watching them on such a close and daily basis but I can only venture a guess the impact that JUST Red Tailed Hawks in rural areas would have on a gamebird population. My lord.
They don't miss ANYTHING. If I saw a small game animal in my yard over the last few years I found myself rooting for the little guy to find some cover because I knew what was coming.[]I've seen more actual hawk kills than probably many people. I know the neighbors don't complain about rabbits eating the flowers.

Combine hawks with other native predators that eat eggs on up to adult birds and then throw in feral cats and no small gamebird has a chance. Without serious predator control small game hunting in PA will NEVER be what it was.