More people get injured by losing gunfights because of a neglected manual safety than without one because of a ND.
Manual safeties take dedicated and continuous training, especially if you only shoot occasionally or you shoot multiple platforms.
The firing line - ain't 3 sec./3 AM/ threat mitigation.
GR
"We have an inveterate dislike of the profusion of safety devices with which all automatic pistols are regularly equipped. We bclievoe them to be the cause of more accidents than anything else. There are too many instances on record of men being shot by accident either because tho safety-catch was in tho firing position when it ought not to have been or because it was in safe position when that was the last thing to be desired. It is better, we think, to make the pistol permanently “ un-safe “ and then to device such methods of handling it that there will be no accidents. One of the essentials of the instruction courses which follow is that tho pistols used shall have their side safety-catches permanently pinned down in the firing or “ unsafe “ position." -- from Shooting to Live
Safeties earned inveterate dislike from Fairbairn and Sykes, and the dislike was promulgated widely due to their tremendous influence. However, let's consider the two modes of failure mentioned:
1st - the safety was unintentionally off. It is easy enough to forsee ND's happening because of a neglected safety. While we may be determined within ourselves to prevent this, if you're the boss of 1200 officers, someone is going to screw it up.
2nd - the safety was on when the gun was needed to fire. That would be a bad situation and I think there's a number of examples of video evidence of just this happening. Still, I would posit two things: 1st, I can't imagine a cowboy drawing his single action revolver and not being considerate of the need to cock the hammer. He'd rather have to be trained not to cock it should he ever be given a double-action. It seems unreasonable to think a person can't be trained in the need to cock or manually clear a safety. 2nd, Fairbairn and Sykes actually recommended condition 3 carry after disabling the safeties. While I believe they would have preferred DA/SA autos or modern striker-style actions, none existed in their day. They chose condition 3 carry over a double-action revolver because they obviously believed the training to rack the slide when unholstering and then to eject the magazine and rack the slide to clear the gun and then re-insert the magazine was more easily accomplished than training to use a double-action trigger (revolver), and to perform fast revolver reloads.
These guys were not stupid. They accumulated more experience than almost any other individuals in the 20th century, and their conclusions have been repeated again and again. They prioritized the speed of reloads. They understood that high round counts were the norm. They didn't have high-capacity double-stack magazines available to them in their day, but they did write, "Throughout this book we have done our best to emphasise the vital need for extreme rapidity of fire. For ourselves, we can accomplish this...most easily with an automatic. The more closely our own pistols resemble machine-guns the better we like it."
I'll repeat that they emphasized the rapidity and volume of fire, and that's why they totally dismissed the single-action revolver, which would have met their preference for a trigger that is easy to shoot with. The difficulty for recruits to learn a double-action revolver trigger, and their lower volume of fire resulted in the overwhelming preference for the automatic.
They wrote in their 1942 book, "To attain the first requirement [stopping power] we should choose a cartridge that represents what we consider a safe middle-course, i.e. with a bullet of reasonably large calibre [greater than .32] and weight, driven at a very high velocity. As regards the second requirement... we have a preference for firing in “ bursts “ of two or more shots. We think that lack of stopping power inherent in the cartridge is compensated for in some degree by the added shock of two or more shots in very rapid succession.... Obviously, this belief of ours implies the necessity for a large volume of fire, quite apart from the desirability on other grounds of having as many rounds as possible at one's disposal without having to reload."
So what do we have today? "Intermediate cartridges" like the 9mm and .40 S&W (and in rifles the 5.56), high-capacity magazines, and high rates of fire. Triggers that are easy to learn, and no manual safety levers to forget and cause an ND. The lack of a safety to remember before firing was mentioned by them, but their alternate manual of arms (condition 3) suggests that was the least of their priorities.
We often hear people debating "how many rounds are fired in self defense incidents," with no clear answer except that most people are skeptical that they often take multiple high-capacity mag dumps. But Fairbairn and Sykes weren't writing for self-defenders. They were writing primarily from a law enforcement background. And if video evidence is indicative, it seems that multiple high-capacity mag dumps are common in law enforcement today. I regularly see officers reloading in video of shootouts, and reported round counts in incidents are often over 100. Examples that come to mind immediately are the shootout with the armed robber on the city bus where the officer's body cam captures him emptying both a shotgun and his duty auto and then most of the second magazine. Another one is the Vegas officer shooting through his own windshield and reloading and firing a second magazine.
If the FBI's frequently cited report that over 80 percent of officer's shots miss the target, it seems reasonable to have and to shoot 15 or 20 rounds or more. The DoD concluded at some point that the side who fires the most rounds tends to prevail, and that was part of their justification for the M-16.
Having written as much, I will also mention that I have some skepticism whether this doctrine is relevant to self-defense for non-antagonists. The popularity of guns like the Ruger LCP and J-frame vs. concealed-carry submachine guns suggests other people also see the needs of the self-defender who doesn't go looking for trouble as different, but others will carry high-capacity plus spares and that's fine with me. I think we can see how prevention of ND's and ease of use are things that apply to anyone.