Yeah, in a real world for grown-ups you would be right. Unfortunately, we don't live in that world.
"Common use" protects what we've got, but stifles future innovation. At least it's better than the emotion-driven arguments coming from the left, which, of course, are just cover for a Marxist dystopia.
We have Justice Thomas to thank for some carefully worded opinions. I don't have the sources readily available this morning, but I would bet that Thomas was careful to word the Heller decision so as to indicate that "common use" is simply one of perhaps many criteria for qualifying an arm as not being "unusual and dangerous".
Muskets, for example, are not in common use today, but I doubt that Thomas would support banning them.