I would also claim that the property I grew up on in Indiana has not changed in 20 years, but we have gone from 3 to 4 coveys on a couple hundred acres to maybe you hear a bird in the spring. I checked out the historical images on Google Earth, which was pretty cool. Maybe trees grew, but fence lines and hedgerows haven't changed.
I am sure I am missing something, but here is my latest reasoning when I reflect back. Fescue in Indiana has been an ever spreading habitat "disease" for quail and other animals. I believe quail are resilient especially when it comes to reproduction. We had a huge field of fescue grass growing up that is still there today. I think in my 20 year observation window, I believe the population struggled well for many years. While I don't believe predation to be an overly huge part of the problem, I do think that in 20 years other animals have been able to succeed more than others such as the bobwhite, thus increasing certain predator's impacts on quail. I don't think it is predator birds either, even though I do see way more hawks now than I did growing up. I think it is more so ground predators that have an impact during nesting and growing seasons. I think our population held on as best they could through the 90s into early 2000, but continued the steady decline to the sad condition it is today really due to this habitat problem that was already here for quite some time.
Hunting quail in Indiana, I remember having a lot more walk-in flushes whether coveys or singles, than what I experience in central and western Kansas. At least in the areas I hunt in KS, the birds tend to run and tend to flush wild as a group. The reason for the difference is that the birds in Kansas have room to run in their cover, where as birds in Indiana do not. They behave a little differently due to their habitat.
Indiana has a wetter climate than Kansas, so things don't struggle to grow and get thick through spring and summer. Not sure about the difference in Missouri climate v.s. Kansas. Three years ago a farm we hunt opening day each year held 3 coveys on about 150 acres. We have found no birds there in the last 2 years. This is a close family friend of ours, and he has focused on planting food plots. One dominant plant he established in several areas is partridge pea, which comes back every year. However, fescue and thicker brush have found its way into these areas. He has never done any burning, and I think this is long overdue.
There's my 2 cents, let me know if I am full of BS and if I need to change my thinking.