Even a draconian law banning the existence of all hi-cap mags would not work for the reasons you mentioned, but there is one more.... Most people have no idea how easy hi-cap mags are to fabricate. I won’t provide the instructions here, but trust me, it is easy.
Make (or print) Your Own AR-15 Magazines
The push to ban standard magazines is insane. It is easier to make a 30 round magazine than it is to grow a marijuana plant. The left is pursuing an unachievable result that will not help stop mass murder.
Rifle magazines are made from plastic, steel, aluminum, or fiberglass. There are 100s of millions of them in existence in the United States. They are easily made at home. They are just a box with a spring. If you have a few 10 round magazines it is easy to convert them into 30 round magazines. Technology has improved to the point that a person can print out functional AR-15 magazines at home.
Before that, magazines were easily made of sheet metal
A reader comment:
Do we want a “war on magazines” to go with the “war on drugs?”
I will. Much like the printing press, this a technology that cannot be suppressed, or, if it is, the cure is worse than the "problem".