Er...you can't drop a vertical line in the way you suggest unless the saddle's level to begin with, which was actually my point.
And leg going off flap - imagine the saddle out of balance front-to-rear so the pommel sits too low. The rider automatically re-balances himself (in most cases) by sitting back, usually pushing the legs forward to compensate. This is by far the commonest reason for legs going over the front of the flap and happens with all saddle types.
See what I mean?
Let me clarify: The "vertical" would, indeed, be vertical IF the saddle were sitting perfectly level. But it it's not, I, maybe not you, can still look at the saddle tilted or upside down and gauge that angle that rider's thigh should be, where the saddle wants the rider's thigh to lie on the saddle flap, and there the stirrup leather should be hanging, if the saddle were sitting level and the leather is hanging straight down.
I get what you are saying about balance. But the real issue is the saddle that does not fit either the horse or the rider ideally. The seat tilts one way or the other due to poor fit on the horse. The rider's knees go off the flaps because there is not enough flap for that length of thigh, when the thigh is where the saddle wants it positioned.
The issue, as the photos show, is that people are using saddles that just do not have a forward enough flap. They can possibly get by with it if the saddle sits just exactly right. Or it it tilts backward, lol. Or, they lengthen their stirrups to ride a straighter flap (dressage rides a longer stirrup than cc. Saddleseat does, too.)
Buying a larger seat size to get more flap may or may not work. If the longer seat results in a seat dip to stirrup leather length that's too long for the rider, then the saddle will chair-seat the rider.
Buy the tree that fits the horse, and the flap forwardness and flap length and stirrup bar placement that fit the rider.