Well, the alternative would be armed soldiers in public gatherings of more than 50 people. Trained guards who have military discipline might work -- but the Posse Comitatus Act prohibits this to federal troops but a state's National Guard can be mobilized.
The Guard's training and discipline is less than optimum. But you cannot put a cop on every corner. I read there were 600 to 800 people in that club -- a target of opportunity shooting gallery for a psycho-terrorist ISIS wannabe
Good thing he was taken out before he killed more innocents.