It isn't hat coins aren't being used and lost, they are but as you related folks are carrying less than we used to. A bigger problem is that a lot of schools and their playgrounds are fence3d off in many locations, and propel, as a whole, are not using public parks as often. Gosh, I remember when I was a kid and I would head to the closest city park, one mile away, to use the playground equipment or play ball or watch the older kids use the baseball fields for games. On some weekdays of a summer, or often on a weekend, you would see others enjoying a frailty picnic at the park, blankets and all.
Too many people getting into the detecting hobby are younger and don't know about or have experienced those 'good old days'. All the activity the popular urban sites enjoyed for decades left lot of coins, jewelry and other interesting lost things to be found. Newcomers will still find stuff, but for those of us who were the early birds and get an active start in this great sport, we had the pleasure of thinning out the bounteous amounts of lost keepers! I started in march of '65 and the 'glory years' were all through the '70s and most of the '80s. But I still have fun giving sites a go. I just try to concentrate on older sites, renovation, permissions, etc.
No, things are not going to be as wonderful in the future and even today it can be downright frustrating. But I knw if I hang in there and do my part, I am sure to stumble upon a nice site or two and that will brighten my life.