Actually, I don't care much about if the IQ can measure the mental capacity precisely or just find those who are mentally disabled. I think the core problem is,

Actually, I don't care much about if the IQ can measure the mental capacity precisely or just find those who are mentally disabled. I think the core problem is, that our society thinks of an IQ as some magical figure to determine the value of a human being. I think an IQ is much closer to what the clock rate is for computers. Just because a computer runs at a higher frequency, doesn't mean that it is computing more important calculations. In fact, there is no advantage in having 3 times the processing power if you are just running a word processor. Equally, there is no advantage in having 3 times the IQ if you are just going to shop groceries. Yes, there are a few use-cases where having more processing power is relevant (e.g. when you have a limited time to make a good and impactful choice), but I think those cases are not a frequent as most people think they are.