From a strict monetary value perspective, you have to find a whole lot of silver coins to equal the value of one gold ring. And gold can be found at any park, old or new.
But, earthlypotluck, it's a matter of playing the odds if you want to find desirable older coins. Contrary to what you said, the odds of a person in the 1970's having old and desirable coins in their pocket was slim.
You can certainly find clad coins and jewelry at a 1970's era park. Also on the plus side, you are more likely to have a less searched area for finding those types of targets because many detectorists won't search a 1970's park.
You can find nickels from the 1940's and 1950's in circulation still today because they made a whole lot of them and, with a few key date and war nickels as exceptions, they aren't rare or collectible.
Likewise clad dimes and quarters from 1965 on have no collectible value and are readily found in circulation.
It's not impossible to find a silver coin anywhere. But assuming no significant human activity before a park being built in the 70's, your odds of finding a silver coin there is long. By the early 70's, there was very little silver left in circulation. Maybe 1-2% of the total circulating dimes at best in the early part of the decade. And whatever the silver dime percentage was, the silver quarter total would be even lower because they disappeared from circulation even faster than the silver dimes. So the odds of a 1970's dropped dime or quarter being silver are extremely low.
Wheat pennies were discontinued several years before silver, but they lastest longer in circulation because they weren't actively being hoarded as early as silver coins were. But by the mid-1970's, wheat pennies had faded from circulation significantly.
Another good thing to realize is a lot of places that are parks now were originally private properties bequeathed by landowners for use as parks. Sometimes little pocket parks will be former residences where the houses have been torn down and the property taken back by the city for non-payment of taxes, etc.
That point was driven home to me a few years ago when my little niece and I detected a tot lot right on the outside of an interstate highway fence. After she shocked me by finding a 1880's Indian Head about 4 inches down, I researched and realized that where she found it would have been right by the driveway of a house that was taken for the highway.