Let's pass a law that allows criminals to only have 10 round magazines...
The limit to magazine size is about trying to at least reduce the body counts of mass shooters. Are you proposing that we somehow try to guess which gun owners will go active mass shooter? If so, how would we know that? Or are you just saying that after they start killing people, we should stop allowing them to have new magazines over 10 rounds? Because at that point, we're generally trying to arrest or kill them...
The distinction between "criminals" and "law abiding gun owners" is kind of meaningless in general, but especially in the context of mass shooters, which is where the magazine size questions are relevant.