I have never heard anyone express the idea they used a manual safety because they could not competently exercise trigger discipline.
Yes, it's quite rare for a person to admit that the reason THEY
personally use a manual safety is because they aren't competent to exercise trigger discipline. For obvious reasons.
But it's certainly common for people to say that problems with trigger discipline (in general, not, of course, as it applies to THEM) is why manual safeties are necessary. That sentiment has certainly been voiced on this thread, even touched on by you earlier in the thread when you commented about how people can't train to eliminate twitch/jerk reflexes or to keep their finger off the trigger in high stress situations--and then again in the post I'm quoting from.
Conversely, if a person can't learn proper thumb safety actuation it makes no sense that they can learn proper trigger actuation.
I would agree with that, but, just to be clear I am NOT asserting people can't learn proper thumb safety actuation. I'm absolutely not making that claim. I hope that nothing I've said on this thread or anywhere else suggests that people can't learn to operate manual safeties because that's certainly not what I believe.
We all know the real rationale for no safeties.
There are more than just one. Here are a few, there are probably others I'm not thinking of.
As you mention, the fear that forgetting to operate the safety properly could cost a person time in a time-critical situation drives some people away from manual safeties.
There are several reasons why people might take that attitude. Some people have actually had an instance in training or at the range where they forgot to disengage the safety--even in relatively low stress situations. Some may have spent a lot of time shooting revolvers and aren't used to manual safeties for that reason. Some may simply not be interested in putting in the extra training to learn to use a manual safety competently on top of learning the rest of what they need to know to operate a gun safely and competently. And then there are the people who just don't want to train at all so they want something that's as simple as possible. I'm certainly not trying to justify all of those rationales, just pointing them out.
Some people aren't so worried about the time issue, but don't want them because they add cost and complexity to the firearm. If a person feels they don't need it, why would they pay the cost to have it, however small? If a person doesn't feel that they need the function, why run the risk, however small, that the parts required to provide it might break or malfunction. The fact that the safety is expressly designed to disable the gun lends some level of credence to this concern. We see this same kind of concern about the gun lock type safeties even though they're far less likely to be accidentally engaged or to break than an easily accessible control.
Some people feel that the presence of a manual safety can cause people to place unwarranted faith in them. That could result in unwise actions. Like pulling the trigger when they shouldn't because the safety is on--or they think it's on. Like putting their finger on the trigger when they shouldn't because the safety is on--or they think it's on. We've all heard the "Don't worry, it isn't loaded." excuse--"Don't worry, the safety is on." is very similar, if perhaps less common. We see this same general kind of attitude voiced by people who say they keep all their guns loaded all the time so they don't ever become complacent.
Some people place a lot of importance on being able to operate a firearm with either hand in case of an injury. Ambidextrous safeties are a lot more common now than they used to be, but even with the large variety of handguns available they still aren't nearly universal. One choice if that's a major concern is to go with a gun that doesn't have a manual safety.
I'm not saying that all of those concerns are necessarily valid (so I'm not going to try to justify any of them), and certainly different people will place different levels of "value" on each of them. I'm just saying it's an oversimplification to assert that there's only one "real rationale" for why people choose guns with no manual safeties.
But enough with the derision of those who see their benefit.
Derision? Like this?
"The argument is always the same stupid mall ninja nonsense."
"The idea that you can train yourself to never make a mistake and touch that trigger when you didn't want to is laughable."
"So I guess you have it all figured out and you'll never make a mistake. Good for you."
"...have been telling the gullible public they don’t need them, and the gullible public bought it hook, line and sinker."
I agree that derision isn't helpful, but I think that just maybe it's a bit more widely distributed on this thread than you imply.