Glucose as a fuel source is required only in brain. The rest of cells can utilize glucose for fuel, too, but do not need it. The amount needed for brain is produced by our own body (thats why we do not die during fasts or on a zero carb diet). Any glucose amount that goes on the top of that must be taken care of by our body defense mechanism or it will harm us.
that doesn't just magically become fat.
Sure, its not magical, its a well understood mechanism of our biochemistry.
It requires a significant caloric excess for it to become body fat.
Its not about calories. Heat energy of a food has virtually nothing to do with that. Sugar becomes fat when cells cannot absorb any more of it for energy, because staying in blood as it is is toxic for the body. Thats why body rather stores it as fat, to protect itself. Fructose/alcohol are the most dangerous, because they cause fat specifically around organs (liver).
There's less and less credible evidence for the carbohydrate-insulin model of obesity or diabetes. The pancreas doesn't just "burn out" in the presence of a high carbohydrate diet. There are almost no people with low body fat who become type II diabetic, regardless of their macronutrient consumption.
If you eat more carbohydrates that body needs (which is any amount of carbohydrates), body has to deal with it - and quickly (because its destroying blood cells and arterial walls). Pancreas does not " just burn out", instead cells simply cannot utilize glucose anymore and insuline becomes ineffective as a signalling to the cells to absorb the blood glucose.
I have tried eating like that years ago, when I was far less clued into actual science. It resulted in gout and inflammation, and I didn't really lose a significant amount of body fat.
There is nothing in the "actual science" supporting veganism over meat, dairy or eggs. Why you personally did not lose a significant amount of body fat cannot be determined from such short information in one sentence, so its hard to say. Malnutrition, toxicity or even anorexia of veganism is hardly a good solution, though.
A high fiber diet is far healthier in general.
Not "in general", only relatively to sugars without fiber. Because it slows down or even inhibits sugar from being released to the blood, where it acts as a toxin.