After fat adaptation, as I understand it, the muscle is more metabolically flexible, having the choice ot fatty acids, ketone bodies (intermediate products of fatty acid metabolism), or glucose. Since hyperglycemia is damaging, even fatal, the muscle must at all times be ready to metabolize a glucose load, but the fact that the process of adaptative glucose-sparing exists shows that the muscles actually prefer to use the fat pathway when possible.
Fat is the endurance fuel, glucose provides explosive power when needed. The fact that explosive power is not needed often, is shown by the low level of glucose and glycogen the body maintains on a low-carb/keto diet. After all, if you are hunting a gazelle, for instance, you need lots of endurance for tracking it and hunting it down, and only a few times during the process will you need to put on a burst of speed.
Again, I think there may be some confusion here. When cells are chronically besieged with glucose, the mitochondria become damaged and inefficient, and the cells become so overwhelmed by having to deal with glucose all the time that they stop producing some of the enzymes needed for fat oxidation.
When we stop with the carbohydrate and the glucose emergency is over, the reason fat-adaptation takes so long is that the mitochondria need time to heal and to produce new, healthy mitochondria, and enzyme production needs to ramp up again. It’s like overwhelming all your firemen with having to put out fires round the clock; when the emergency is over, they need time to get back in the groove of making building inspections and conducting fire safety programs. But once they are back in prevention mode, it doesn’t mean that they’ve forgotten how to fight a fire. (Not a perfect analogy, but you know what I mean.)
My understanding is that adaptative glucose-sparing (physiological insulin resistance) should be considered as being more a choice not to metabolize glucose (when choice is possible), not an inability to do so when necessary. The muscles can lose their ability to metabolize fat, if we go too long without allowing them to use fat, but they never lose their ability to metabolize glucose, because too much glucose is so deadly to the rest of the body.