The "if we restrict or outlaw something only criminals will get it" argument wears incredibly thin after a while. We're not talking about Prohibition-era alcoholic beverages or modern day narcotics where most of the danger to the human being is to themselves by abusing the substances.
Guns aren't booze and they aren't weed. They can kill dozens of innocent people in a matter of seconds and the corollary that if we ban high-capacity clips, magazines and ammunition drums or assault weapons that "only the criminals will have them and then things will only get much worse" is palpable crap.
Even Larry Pratt of the gun lobby admitted that gun crimes with outlawed weapons fell by a full two-thirds in the immediate aftermath of the passage of the Assault Weapons Ban in the '90s (though I don't think he really meant to admit that on television in front of an audience of millions....it was a rare slip-up from one of the "arm everybody and sell more guns" crowd).
The "only criminals will have this-or-that" counterargument is just a tactic used to try to stop the debate and it does work with some people who are more prone to just give up and feel there's no point and that nothing concrete will ever get done.