I agree with you in part about depth however there are plenty of ways for objects to get deeper than what VLF's can now detect effectively. On the problem with finding nonferrous items in trash, especially iron objects like nails, motion detectors have a long way to go before we can say without reservation that we have hunted a place out. However I think that expecting a detector to signal on a coin under a alum can is close to impossible with current or foreseeable technology. I think 12" on coins in regular type ground where current detectors may not give a signal after 8" or so would be quite enough. Yes, I agree, most coins are within the first 6" or so, yet I have dug clad coins at 7" or 8" and found silver coins almost on top of the ground so there is no way to know what the depth of a coin will be by how long it has been in the ground. How do coins get deep in the ground, these include, mole holes, livestock pushing in the ground that is muddy, fill in, heavy machinery doing dirt work, dirt washing down to lower sections of ground, these and others not mentioned can be factors no matter what part of the country one is in. If you are finding any coins on the fringe of the detectors usable depth then it doesn't take much imagination that there are coins still deeper, yes I do understand there may not be as many by percentage. So I am in total agreement with you on the more trashy areas and the need for a detector to see around some of that stuff to get the goodies, the more trash, the more traffic, the more likely something good is there.
I think my main idea was that detector technology is static for now and has been for some time, that is the reason some of us haven't sold our old ones and went out and replaced them with the newer ones, much like many do with computers, etc.
It is not that I think the newer ones are totally inferior or such but they really don't stand out to the point where the older ones are now all obsolete.
Keep in mind this is just my opinion.
BTW, Whites did "hold back" a few decades ago before they released a new technology but I doubt any brand will do that now, it might mean that they would loose sales!
I like your choice of detector too!
HH