Lots of good details of the reasons for decline. But I think everyone of them are directly attributable to one thing; TOO MANY PEOPLE! That causes a lot of things to occur, including huge increases in urban and suburban population. These urban and suburban people then are influenced by those things many of your have mentioned in addition to the loss of hunting grounds.

I'm sorry to say, I see no realistic solution on the horizon to stem the decline of hunting in much of the US.