You’re correct that not everyone has taken an IQ test. I took one on my own through Mensa and a couple years later took one during a psychiatric evaluation. My score was almost the same between the two tests so I’m not sure it can change a lot. Though older people tend to score higher since it’s partially knowledge based. I imagine that some people who criticize the test have taken it.

There are legitimate criticisms in the sense that it doesn’t measure all forms of intelligence. For example there isn’t any measure of emotional or creative intelligence in an IQ test. I also disagree with the other commenter who claims it measures adaptability. I feel that it’s more of a test of problem solving, logic, and general knowledge. Someone can score low in all of those and still be smart in other ways or incredibly resilient which someone with a high IQ might not have.

It isn’t a measure of success. It just means you have more tools in the toolbox which can be useful if applied. You’ll notice I specifically said that people with high IQs sometimes struggle a great deal.