But it's not only sugar excess that makes that damage, primarily it's the lack of nutrients in processed foods that causes a gradual mitochondrial decline.
For instance, most people consume white sugar because it's cheaper. On the other hand, sugarcane consists of the same glucose + fructose combo as the white sugar but it also contains vitamins, giving us much better chances to remain healthy.
Fats still have some uber-abilities compared to the glucose though. The main one is the ability to "shortcut" the faulty metabolic pathways.
I will give an example. When glucose pathway is broken in a cell, it leads to inability to produce enough Acetyl-CoA in the given cell. Acetyl-CoA is an intermediate substance that is used by mitochondria to produce the ATP. When there is not enough Acetyl-CoA, the ATP output levels drop, causing detrimental effects on health and general well-being. Such cells start starving, degrade and eventually die.
When beta-oxidation pathway is broken, it leads to nearly the same defect, but this time for fats. However, if a person still has at least some number of cells that can do beta-oxidation correctly, those cells can "share" their Acetyl-CoA with other cells through the blood stream. This happens mainly in the liver thanks to the fact that the fats are so powerful that they cause a huge abundance of Acetyl-CoA. And that excess finds its way from the liver cells' mitochondria into the bloodstream, feeding other cells by "shortcutting" their broken glycolysis/beta-oxidation pathways.
The excess Acetyl-CoA from liver gets delivered to every single cell in the body, including the brain cells. Even if a specific cell has subpar beta-oxidation or glycolysis abilities, it still can consume the external Acetyl-CoA without any problem and produce the adequate amounts of energy from it to keep the cell healthy.
That's why a low-carb high-fat ketogenic diet has so tremendous effect on some health conditions - it allows to "shortcut" the subpar metabolic pathways in affected cells allowing them to survive and even heal over time. This is a natural cycle of the body and suppressing it with a sugar too often will cause serious problems over time.
Regarding high-carb low-fat diets: probably they can work given enough nutrients (minerals and vitamins) quite well, probably stretching those 10-20 years of adequate glucose metabolism well into 60-80 years. But still, you better be doing some fasting (or keto) to heal the accumulated damage, at least several times a year.
The obesity is a factor but it is not the cause. For example, Asian people may appear thinner than Westerners, but they are ridden by beriberi more often. (Beriberi is a metabolic condition that is caused by mitochondrial damage due to the lack of vitamins)
Regarding what blocks what it's a complex question. Glucose metabolism blocks lipid metabolism and vice versa for sure, but I have no specific data. From my own observations, glucose is much more potent in blocking the fatty metabolism than the other way around.
Anecdotally I have a much easier time handling carb-rich meals since mostly eliminating polyunsaturated fats from my diet. My previous experience was some combination of “food coma” and “noticeably hangry” a few hours later, but now I mostly just feel energetic for a few hours.
Some of the most metabolically-healthy populations in the world eat mostly starches [2] (and maybe 1–2% of calories from PUFAs), so my hunch is that eating low-carb is more of a workaround for widespread metabolic dysfunction than a requirement to be metabolically healthy.
[1] https://fireinabottle.net/the-ros-theory-of-obesity/ [2] https://fireinabottle.net/pontzers-burn-and-metabolic-rate-m...
(It’s worth pointing out that both of these links are from a site that also sells supplements, but they do link back to the primary sources)