CICO doesn’t work across individuals for reason you outline (just because I lose weight with a certain level of calories, doesn’t mean you will, etc.)

But for a single individual, working to establish your base metabolic rate by counting every calorie and weighing yourself regularly, you can always find a way to be successful with the CICO model. If you establish a baseline that your metabolic rate is (say) 2000 calories a day, then eating 1600 calories a day will cause you to lose roughly a half-pound to a pound per week. It does not mean that every individual’s baseline metabolic rate is the same.

As you also outline, your base metabolic rate also changes as your weight does, so re-baselining by continually measuring caloric intake and your weight to recalibrate is always going to be necessary.

Given the above CICO almost seems tautological: Measure the weight loss you actually observe at a given calorie level, and that tells you… how much weight you will lose at that given calorie level. So it is almost useless as you describe.

BUT, the main reason I’m still a proponent of CICO is that it simplifies dieting away from thinking about what you eat, and instead focuses on how much you eat. If I establish that I need to eat 1800 calories a day, I can still have a Big Mac meal and drink some beers. But I may not be able to eat much else that day. That alone makes it useful for me. What isn’t useful to me, are diets that essentially limit the type of food you eat and tell you to just go nuts and eat however much you want: I will often find myself hating the foods I’m restricted to, and the quantity I eat may end up being such that I gain weight anyway.

To me, CICO is simply a technique that says “actually measure how much you’re eating”, no more and no less. That is what makes it a useful tool.

I am skeptical about reports of people who don't seem to gain much weight despite reading a lot of food. Most of those reports are merely anecdotes, or at best observational studies, and we know that most people are terrible at accurately tracking their caloric intake. More rigorous studies that involved putting subjects into locked environments and carefully measuring their intake have consistently found that the CICO model is accurate.

However, as a practical matter for people who want to lose adipose tissue there are likely more effective approaches than CICO.

CICO works quite well at the extremes. It's effectiveness varies, but it is directionally correct.

If someone is sitting at a 35+ BMI, then their caloric intake is so out of whack, that CICO helps ground them. Similarly, an anorexic who starves gets a clear directive for increasing weight by increasing calories.

CICO also promotes calorie general calorie comparison. It helps you understand that a milkshake has the same weight impact as an average Mediterranean lunch bowl.

Now the absolute impact on both foods will vary from person to person. But comparatively, it helps ground the unhealthiness of foods like sugar rich shakes and icecream.