That's rather serendipitous, as I was just at a foraging talk yesterday and asked the expert about this, and though he didn't have any specific info, he had some of the same theories.
Twenty years ago I used to be able to find wild plums all over the area where I grew up, but haven't been able to when I went back to those same places in recent years. I'm betting that they just got crowded out by invasive plants like buckthorn. That's pretty sad.