It's also still bad for your teeth since it's acidic and has some unclear negative effects on gut bacteria.
I don't buy the gut bacteria effect, or at least I haven't seen anything clear that's positive or negative - just vague comments that it's bad.
Maybe the taste of artificial sweeteners encourages people to eat more, but as long as you don't then the diet soda is fine.
There was a famous story of a man who lost weight eating a calorie deficit of twinkies. And it worked! But it’s also hard.
So the real question is whether diet soft drinks make it harder to feel sated eating an appropriate amount of calories.
The main finding from an admittedly quick look is that drinking artificially sweet soda may encourage people to eat more calories and gain weight. If you're controlling your calorie intake then that's irrelevant.
Outside of that other claims seem highly questionable.
I push back a little on this, because most of the time I've heard people say diet soda is as bad or worse than regular it's because of unsubstantiated woo. Is drinking only water better for you? I'd guess almost certainly.
Is diet soda particularly harmful? I doubt it - and it's almost certainly better than regular soda (or all of those high sugar fancy juices). It can also be really helpful when losing a lot of weight as a zero calorie snack.
It's relevant because controlling caloric intake without relying on the bodies natural hunger signals seems to be close to impossible for most people. The alternative is to ignore your gnawing stomach all day and carry a food scale to every meal. Only the most dedicated athletes can sustain a routine like that. Everyone else eats until they are satiated and will gain weight.
There's an obesity epidemic in the US and discounting mounting evidence of a substance modifying our eating habits in a potentially unhealthy way as "unsubstantiated woo" strikes me as counterproductive. We should be taking a very critical look at the American diet as something is wrong.
On an individual level where diet soda is replacing regular soda, or where diet soda is used as a treat when controlling caloric intake otherwise I find the generic “diet soda is bad for you” to be misleading enough to be wrong.
The “woo” is that usually it’s not some commentary about artificial sweeteners increasing caloric intake, but some nonsense about artificial sweeteners being unnatural or causing cancer, etc. it’s not too dissimilar from 5G warnings or anti-vaxx.
Large observational study referenced in this article:
https://www.psychologytoday.com/au/blog/the-red-light-distri...
Some decent primary sources in this article, specifically interventional animal model studies:
https://ketodietapp.com/Blog/lchf/artificial-sweeteners-the-...
This interventional study was ended early on ethical grounds because the depression caused by diet soda was too severe: