I think you can reach a critical mass of armed everyday men and women. I don't know where that lies - 1 in 4, 1 in 3? At some density it will not be possible for the perpetrators to watch everybody all the time well enough to keep from getting shotSeems cyclical. The pharmacies started hiring guards for a bit then dropped them pretty quickly. One guard will dissuade the strong armed robbers (generally) and the not very dedicated. A lone guard is a target for the dedicated, and I've seen multiple instances of them dead/disabled/disarmed/taken hostage over my career. Multiple guards is a stronger deterrent, particularly if not allowed to be in one cluster but within view where one ambush style assault can't immediately target them all.
The biggest barrier to state side security is pay. It's tough to recruit and retain highly qualified candidates for the wages they tend to offer and there is no budget or will to train up new hires who didn't bring their own skills. Higher wage jobs are often hiring off duty sworn law enforcement, require a security clearance, or are longer term .gov gigs.
The caveat is the 'no guns' signs probably need to be outlawed (most of us are not as fortunate as IN), prosecution needs to be relatively hands off (if it is a necessary decision to kill or maim bad actors to protect yourself or other innocent people, worrying about being bankrupted or jailed should not figure into that tactical assessment), and people in general should have it driven home to them that reality is nothing like TV - that if you draw you had better be willing to use your weapon, trying to hold several bad guys at gunpoint like in the movies is going to get somebody killed, most likely you
If your 'little angel whose smile would light up the room' is killed during commission of a violent felony, you should not be allowed to pursue a civil case. I'm tired of the family of perpetrators whose kin are killed by police or Good Samaritan treating the occasion like a potentially winning lottery ticket