IQ tests measure the g factor and also happen to be the best way to measure it. So those opposed to IQ should either come up with a better way to measure g or be open about their rejection of the existence of differences in cognitive abilities.
Ah, yes. The g factor: “that which is measured by IQ tests”.
The main arguments for IQ tests, as far as I understand, that they correlate with an assortment of cognitive metrics, which all correlate with each other. This is a question. “It's the g factor” is not an answer; you've just stuck a circularly-defined label on the question (or, rather, on a particular frequentist analysis of the question) and declared it solved.
Do you see a problem there?
The g factor—or more descriptively—the theory of general intelligence has little support outside of the field of psychometrics. And the field of psychometrics is not known for it’s scientific rigor. Needless to say, there is little reason to believe that the theory of general intelligence has any grounds in reality, nor that the g factor exists outside of an operationally defined concept that might not hold any descriptive power for human behavior outside of a narrow setting of pen and paper tests.