I find your 40% rationale interesting.

#1: There are people eating 100% fat diets. Many Atkins eaters are below 10% carbs and doing fine. I am not sure where that puts the fats and proteins. But it is hardly a reason to eat less (more) fat.

#2 Triathlon athletes are doing mostly aerobic training. The issue for all athletes is getting enough oxygen and energy to their muscles. Most athletes run out of oxygen before energy. And there is some suggestion that people who are burning fat (in ketosis) are better able to extend their endurance levels.

#3. I doubt if any of that is true or that anyone has ever suggested that is how being in ketosis works.

#4 Most fat is metabolized in the liver, not the muscles. Technically fat has more energy per pound. The major hurdle in burning fat is that to completely convert your body, it takes about 2-6 weeks ( that induction period ). Anyone exercising during the induction phase would have noticed the drop in energy/stamina/endurance. Then after this induction period, your stamina/endurace return to "normal".

You are assuming that ketones are an important component of LC food programs. Actually, ketones are part of a starvation scavenger pathway to supply the brain with energy when the body has no other food source. Most likely fat is being metabolized for energy OR fat is being converted into glucose by a yet unknown undetected metabolic pathway (my unproven scientific supposition/theory) Your blood sugar does not drop while burning fat after all.

Carbohydrates are NOT an essential food source. There must be a way to live without it.

A lot of training practices are anecdotal, passed down verbal from person to person. Often what is considered good is just plain wrong. A case in point -- stretching before exercising. Is it useful? Turns out, no. A study showed that the best warm up is to use the muscles in a manner like you are going to exert them. Jogging is a good warm up for racing.