Sorry man, you are dead mistaken. I live in NY state and we have plenty cold winters here. I have been hunting for over fifty years and 90% of the coins I find are in the top 6" of soil in undisturbed sites including colonial coins that have been in the ground for 200 years. At tub full of dirt and worms is not reality by any means. I use fine detectors that will easily hit a dime at 10" and I do find those but in areas where dirt has been added through landscaping. Think what you like, many with many years experience will agree with me, coins don't sink. Even in the sandy soil of Cape Cod I find colonial coins at 6" or less unless in the woods where leaf matter causes dirt build up faster. A jar full of coins originally buried at 1-1&1/2 feet will not sink one centimeter. Yes, I know from experience but we don't talk about that.
Ummm, no, i'm dead right, it's simple biology of soil. Objects absolutely sink in certain types of soil Rates of sink depend on your soil type, clay content, sand content, gravel content, insect and rodent content, amount of rainfall, make up etc. Soil is a cycle of life constantly turning, pulling up nutrients, sending roots down, that decays, that in turn get pulled up again. Soil for the most part, stays a constant depth. Did you even read my post? Bigger objects don't sink as fast (sometimes at all) due to their surface area, but smaller items can and do readily sink . Deposits on top are not the only reason why you find a coin six inches down 200 years later.
By your account if the only reason your coin is 6" deep at 200 years old is deposits on top let's math that out, over the past 10,000 years, your soil would be 25 feet deep! But it's not, is it? It's probably about 1 foot deep before you start hitting clays and gravel. Soil is a constant depth, about average of 1 foot deep across the earth. It doesn't grow deeper year after year (in certain isolated cases, yes, like erosion, wind and flood deposits etc).
Add to the mix roots and rodents and sink rates can really accelerated. You won't see as much sink deep in the woods with a full canopy and little underbrush too as there is not as much small plant root growth and less insect activity.
Edit: Your jar buried 1.5 feet deep doesn't sink any further because its already past the "alive zone" of soil. It's down past most insects and plant roots and into denser material with more clay, gravel and in some areas, is even getting down to bedrock. Again, it all depends on your soil type in your area.