You can't practically carry an AR15 concealed. If that is the weapon you need, then it must be open carried.
Further, your statement doesn't consider the tactical advantage to YOU when others carry openly.
It is exceptionally rare when you need an AR-15 in any sort of urban or suburban situation away from the home. Though I’ve no objection to it, its optimal range is outside a typical gunfight, so any need to carry it concealed is mitigated.
Likewise, criminals only very rarely carry openly, and there is little or no need for advantage against an honest citizen carrying openly, unless they enter into an existing gunfight and do not know who is the criminal and who is the good guy. You still don’t want to shoot them, you want them to not shoot at you.