Just knowing that the average is 50 does not in any way tell you the grades of the majority of the class. Most of the kids could be passing while a few abysmal performers bring the average down.
That’s what you take issue with? That the analogy isn’t completely perfect? Do you get the general idea of what I’m saying, or does that slight inconsistency complete negate the entire argument?
The point is that an average isn’t indicative of overall health… it’s just a value representing the average income. It makes no bearing on actual economic health without comparing it to other factors.