Well in theory, you can't do that now anyway with the top of the tackle zone being the line of the shoulders. When a defender tackles a player who is picking and driving in the way you describe, they invariably tackle the player at the top of the shoulders, which is technically illegal under current law anyway, so I can't see that lowering it makes any difference at all. Referees will still do what they do now with such tackles; apply materiality, recognizing that such a tackle is a low risk for injury to the tackled player.
The rationale here is to cut down on the number running, upright or standing ball carriers who cop one under the throat or in the head. By lowering the top if the target zone to the line of the armpits, you make those tacklers aim lower, which means they are less likely to strike the head and neck,. Also, a lower target zone would result in less chance of a tackle "riding up" to the head/neck area.