Silver and old stuff dont grow in the dirt like worms and grass, when dug up there is one less.
But go to your site, any of then. Stand there and think, can you honestly say-- The last silver is dug here-- and leave. No you cant.
Probably there is many coins left and is it a big outhunted site, probably several hundreds or more.
Problem is to find them, and to do that you have to know they are there ( believe) and have the correct tool and attitude.
Do you ever find a trash object from the 50s there at 4 inches, maybe you did not even dig that 4 inch trash item, Never knowing what was below it in the dirt.
My prediction is that some of the best finds are still in the dirt, masked or too deep to quickly pass with modern detector. You need to dig trash, hunt slower then you think is too slow. There is no armageddon just a change of times and metode to detect the goodies.
The Explorer can see deep, very deep- but with the deepest signals come the use of large coils and minerals to alter IDs and trash to alter IDs too. I have passed many goodies and so have U, lets go get them the next time.
EDIT: a tip to go- Hunt during sunset , the lighted period when the sun is down or at night if u can and dare. No sun interference WILL give you a few more inches and lets the detector operate at its best, both discriminating and ID capability. That is the truth!!
bfodnes