[1] https://www.uclahealth.org/news/observational-studies-shed-l....
And if consumption of diet soda correlates with other things, for example an unhealthier diet in general that the person "compensates" for by only drinking diet soda, then it would show up in an observational study even though there's no cause-and-effect.
But remember, diet soda is 99+% water. Really the only hypothetical problems with diet soda could be 1) artificial sweetener chemicals in the body triggering cancer or other ailments, and 2) artificial sweeteners triggering a glucose-like response triggering weight gain. But studies with rats indicate 1) shouldn't be an issue at anywhere near the concentrations in diet soda, and 2) seems rather far-fetched.
It's not what I do personally but I also don't judge, because scientifically, it's basically just water.
Do you have a problem with somebody drinking 2 cans' worth of LaCroix in the morning, 1 at lunch, 2 in the afternoon, and 1 at dinner? Why should that suddenly become "ridiculous" because an eyedropper amount of caramel color, caffeine, and artificial sweetener get added?
We're eating and drinking acidic things all day long, orange juice and grapefruits and the rest. And diet soda doesn't adhere in a layer to your teeth, the way things like orange juice and sugary Coke do.
Our enamel repairs itself through our saliva. Otherwise you'd never be able to drink a lemonade or eat a grapefruit. The acidity of Coke Zero is a nonissue in the context of a normal diet.
(Not to mention that flavored carbonated waters like La Croix and Bubly are also acidic. Bubly grapefruit flavor is pH 3.86, while Coke Zero is 3.18, from a quick Google.)
Sugar, and the addition to things that taste like it is far worse than we understand. It seems to make people do crazy things.