I agree with you that the real evidence lies in the soil. Crop marks in the general area need to be checked out, and the lack of casual finds is significant. One would have expected a sudden spike in those after WW2, when the increased use of mechanical traction geenerally resulted in deeper ploughing than was previously possible. Field finds are still surfacing from other human activities from a wide range of dates.
On the other hand, I don't find it particularly rational to build too much on discussing the modern photographed landscape for possible battlefields because of the magnitude of changes that may have occurred. Roads probably tend to be stable, if only because some local landowner is going to kick off if someone starts beating a new path through his patch - but woodlands... after four, five hundred years, the result of traditional timber and firewood harvesting would be pretty indistinguishable from similar activities prior to Agincourt. A n arrow gap between copses could increase massively over a century or two
Remember Wellington's reaction when he revisited Waterloo - "My God! They've destroyed my battlefield!" (And converted most of the casualties into bone meal in the process...)
Not looking for argument - just registering a viewpoint