Mr. helton, I find it hard to believe that in the state of Michigan, that you can't find more productive parks that give up easier silver than that. Either you are un-willing to travel around to sample other places, or you're just not proficient at picking out deep high conductors from turf.
As far as "just this one park", yes I can envision parks that are "no longer worth it". There are parts of my state (for instance) that got heavy pressure back in the 1970s/80s ("silver rush" era) that are now nothing but a junky blighted mess, and simply not fun to work anymore. Yet there are other parts of the state that saw less pressure, and we can still reliably go to, to get deeper silver from them.
As for the tactic of strip-mining to get deeper or masked things, you're welcome to try that. But if it were me, that would simply be a park that I would pass on. I mean .... seriously now .... do you really think you are going to strip-mine a turfed park, and not raise the ire of some busy-body gardener ?
I recall a fellow, who had gotten scores of silver coins from an old park, back in the very early 1980s. At a certain point, he began to wonder how many nickels, or gold rings, or masked higher conductors he'd been missing. So he gridded off a certain zone (which had been particularly nice to him in earlier years). He "made it his mission" to dig "every last target", assuming that there MUST be nickels he'd been passing, or targets deeper than his disc. allowed (or masked, etc...). And for a year, he dug every single signal. He kept copius records and charts. He'd go a few times per week, after work, and dig out another 30 to 50 targets, keeping accurate counts/records. By the time it was done, yes, he had a "few deeper silver", and a "few dateless orange cruddy V nickels and bufallos". But his conclusion was that it was simply not worth it. he had HUNDREDS upon HUNDREDS of junk items. And he felt that a) if gold jewelry were his goal, he was simply better off heading to the nearest swimming beach, and b) that the dateless orange/brown cruddy nickels also weren't worth it.
But it would depend on the park, and how deep, etc... Obviously Michigan can have parks that way pre-date CA parks.