Whether a person believes we "need" a 30 round mag or not, they're not going away, no matter how illegal .gov makes them. There are millions in existence already. More will certainly cross our incredibly porous borders and then there's the specter of "3D printing" mags.
Nope, it's a pipe dream.
So, given that scenario and the fact that criminals are notorious for ignoring the law, why does anyone want the bad guys better armed than the good guys?
As I've been told, "No one's that been in a gunfight has ever wished for a smaller gun and less ammo."