What would the world be like if there were no tobacco, no alcohol, no drug abuse and no mental illness?
Pretty damn boring? Well, there's the first difference. You need such a super-executive. Something that can replace the existing basic heruistics for developing the brain with a heuristic that contains a lot more formal logic and testing.
If there is no drug abuse this entails that all the drugs we do use are used in societally acceptable ways, and if there is no mental illness then there is no danger represented by such usage. There has never been a drug-free society and there never will be. What, really, is the difference between coffee and marijuana? That society says coffee is okay, but marijuana is bad. Why? because the common perception is that coffee increases productivity while marijuana decreases it. Well guess what, the people who built your house very well could have been high, the people who make your food at a restaurant are probably high, and the people who make all of your entertainment are definitely high. Why don't we instead ask a more realistic and reasonable question, what would the world be like if people weren't threatened with legal consequences (which have been shown to have detrimental effects), and instead offered treatment (which have been shown to be effective) when their usage becomes abusive, and, what if society did not stigmatize those who use, those who abuse, and those who seek treatment?
Tobacco is a puzzler. There is plenty in nature that is deadly at levels one would normally regard as barely detectable. Some are deadly at any level, but others become absolutely vital for life. It's very hard to predict which something will be. Tobacco has no obvious benefits, nicotinoids do not have any positive roles that I know of, but that's the danger with human biology. Absence of evidence isn't evidence of absence. You need to be careful when making assumptions. All I can say is that at any dose any dispensing system yet developed provides, any benefits are totally masked by harm.
Mental illness is an inevitable byproduct of brain evolution and brain fluidity. A lack of both of those would make life very amoebic. However, let us say that there is some constraint such that neither mutations nor soft adaptations can occur in a way that would permit mental illness. In other words, microevolution is impossible, you have to have macroevolution, and the brain has a super-executive function that can systematically retune synapses and alter the connectome by calculating a defect-free solution to the combination of existing state and new data.
Well, there's the first difference. You need such a super-executive. Something that can replace the existing basic heruistics for developing the brain with a heuristic that contains a lot more formal logic and testing. Brains would have to be substantially larger, you've replaced a simple heuristic with an entire engineering lifecycle and you've got to have that lifecycle performed in precisely the same amount of time as the original heuristic, including all latencies for relaying information around. You'd therefore be spending a lot more of the day meeting nutritional needs. As one function of sleep is to wash out toxins from brain cells, you're also going to have to sleep more. That means far less of the day can go into anything productive.
Alcohol is more complex. A low intake has health benefits, but nobody knows why. The mechanism is not understood and experiments so far conducted have been too simplistic to unravel the complexities. There are other things in alcoholic drinks that are healthy, but not apparently in isolation. Biologists are very unhappy because you can't model complex interactions easily and it messes up the calculations something chronic. They probably need more alcohol. Anyways, the total absence of alcohol would increase health problems and shorten potential life expectancy.