Fat vs. CHO utilization varies wildly from person to person. It's both training and diet related, not just intensity related.
Alan Couzens has lots of real world data on this.
Here are some good reads on this topic from AC:
https://www.alancouzens.com/blog/fatburning_test_2.html
https://www.alancouzens.com/blog/imp..._burning1.html
https://alancouzens.com/blog/improvi..._burning2.html
I think most people would argue that if you are involved in endurance events lasting more than 2-3 hours, it's of great benefit to be as much of a fat burner as possible. Nobody (well, not many people) is saying to not consume CHO on the bike, it's just that keeping up with taking in CHO to compensate for the CHO that is burned can be difficult. And the longer the event, more difficult it is. You can only store so much, and your gut can only process so much.
It all comes down to having a healthy metabolism. If your body can use fat as fuel, it's healthy. Not only does this have athletic performance benefits, but it can prevent T2D, and if you have it, reverse it. T2D is not a permanent condition. Undo what caused it and your body will heal. I should note that having defective aerobic metabolic machinery is starting to be implicated in all sorts of other diseases too - cancer, dementia....
Low carb diet and lots of zone 2 cycling (at or below LT1) is the basic prescription.