This is a very valid "point" or observation that hits hard for me.
In my County of Stewart, at least the main county between the Tennessee River & the Cumberland River, there is essentially no agriculture, no chicken manure being spread, and no chicken houses. In fact, a big part of this land mass is the LBL National Recreation Area.
Turkey populations in most of Stewart Co. have been on a steady decline the past many years, despite many areas experiencing improving nesting habitat. Same has happened with bobwhite quail, which have now become almost totally extinct, yet I could find huntable quail populations only a decade ago.
At the same time, I see some areas in West TN where chicken manure is replacing other fertilizers, where commercial chicken houses dot the landscape, yet turkeys are expanding.
I truly believe whatever we're missing, some missing factor(s) greatly contributing to quail's failure to thrive (despite improving known factors to benefit them), will turn out to be the same factor(s) greatly contributing to the turkeys' failure to thrive.
I don't profess to know what these factors are, but do believe perhaps the most under-studied, and seemingly unrealized factor is increased predation by a steadily increasing raptor population that has been evolving to focus hunting more on wild turkeys.
Unlike many, I do not believe coyotes are much of a factor (at least not in most of Stewart Co.) because we had similar to higher coyote populations over 20 years ago as now, and in many these areas we have fewer coyotes than when turkeys were thriving. Coyotes are a factor, but imo an over-stated one, not a primary factor in the decline.
Chicken houses & chicken litter could be a factor is some areas, but there is no proof that they are.
There is proof in many areas that chicken houses & chicken litter is NOT a factor.
One very specific very large isolated land mass is LBL.