It's actually "genius" that's the problem. There aren't enough geniuses to affect a trend on their own.

You could swap "plenty" to "every" and it would still have the same problem

There are "plenty" of cold days as well. Does that disprove global warming?

Not only is "plenty" not an objective quantity, but even an objectively large quantity is irrelevant if it's not a large percentage of the whole. 100k geniuses with addiction problems wouldn't disprove the rule if there are 1b people with addiction problems who are of below average intelligence.

As he should. Plenty is extremely vague. It definitely means more than 1, probably more than 3, but after that.