It depends on where you are geographically located. Believe it or not, some places were already getting a lot of md'ing pressure by the early 1980s, so it was no longer as easy as you depict it, by the mid or late 1980s. It all just depended on the skill of the hunters in the area, how much pressure they put on a park, what type units they were using, etc.... There had been a dealer here in my town since the early 1970s, and 2 clubs in two neighboring towns, with some die-hard hunters. But then in some other towns a few hours away, I noticed that hunters there, who were still using TR disc. in the early '80s (as opposed to the deeper seeking motion discriminators that were coming out then), that those parks still put out silver in easier quantities. Especially if the "first generation" that had peaked out at 4 or 5" deep, had essentially thinned out the clad for us
I think the real hey-day was the very late 1970s, for the first guys to get the ground-breaking 6000d, and Red Baron SPD. They got an immediate inch or two more than the next closest discriminator, and covered ground fast. But yes, some geographic places didn't get a lot of good turf hunters till the later 1980s or whatever. Heck, I bet there's still some small podunk towns, that would mimic your 1986 experience, even today
A friend of mine stumbled on a park in a small town in Idaho, where, in a week, he and two friends pulled over 200 silver coins from. That was just 3 or 4 yrs. ago. All depends on the caliber and tastes of hunters in each locale.