First of all, think of wine. Fruits are different year to year. I think this has been an especially good year for local peaches. Next year might not be as good.
Now, plums, I cannot speak fully to the problem, but I know at least one local farmer who stopped a few years ago (now) with the Stanley prune plum, i.e., the standard purple plum. His problem, he said, was that people picked and ate them too soon. That the plums eventually ripen to high sweetness, but until then, well...and too many people always stayed in well territory.
I'm guessing some of it is the season, some the types of plums.
Think Yiddish, Dress British - Advice of Evil Ronnie to me.