Quail have a lot of predators. Coyote, raccoons, snakes, bobcats, foxes, turkeys, squirrels, you name it. Not all of them can fly. Predation is greatest at the nest -- eggs or chicks.
I think the rest is pretzel logic. If there was habitat, quail would be plentiful, coyotes, the same ones you claim rarely eat quail, would eat quail, but it wouldn't make a difference, because of the habitat?
I've read a lot on the whole quail thing, and it's not completely a habitat issue. It's the single issue people latch onto, but there's more going on. The whole mix of wildlife has changed over the last 50 years. Turkey and deer are back in abundance. Quail are down. I tend to think there's a connection.
On the habitat side, you're correct -- plowing to the road is a major factor. But, pesticide and herbicide use is up, pastures have been converted to fescue. Quail eat bugs and seeds. CRP is reverting back to cropland at a very fast pace due to reduced payments and an incentive to grow corn on every available patch of land.