As soon as we start trying to answer why we "need" more than ten rounds we have lost the battle.
Nobody "needs" most of the stuff we own. As Americans, we have a right to purchase and own whatever we can afford.
That plays well when you're preaching to the choir.
If you're trying to sway a fence-sitter, you may have better luck if you start by proving it is possible to demonstrate a need and then work around to the real crux of the matter.