You seem to be claiming that there is no possibility whatsoever that diet drinks can contribute to weight gain and even obesity. You may have reviewed both sides of the question and come down on one or the other. But I find it lacking in credibility that anyone, much less someone with 25 years of school, related work experience and research, apparently on this very issue, doesn’t proceed with at least a hat-tip to the evidence contrary to your expressed view and, more importantly, to the plain fact that in all scientific matters nothing — nothing! — is ever finally, once-and-for-all settled.
Then there is that little matter of the complexity of the human organism and the fact that, not only is each human body different, but each human body differs from how it operated previously at different times during its life cycle.
To act as if it is so absolutely settled that diet drinks do not cause health problems, for anyone, is a little excessive and not really very scientific.