Well, the mild acidity is bad for your teeth. Other than that there doesn't seem to be much that stands out as an issue, at least from what we know so far.

Can it be called mild? Diet coke has a pH of 3, which is the same as vinegar

There seems to be some suggestion that the sweetness triggers digestive system changes that could be harmful: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3772345/

It’s very vague though

1. There seems to be some suggestion that X

2. ???

3. In no world can I imagine ¬X

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7014832/

[1] shows that artificial sweeteners lead to higher insulin resistance. It is so because they provoke insulin level increase due to their sweet taste.

Caffeine is a tricky one.

If you can drink 6 cups of espresso a day, you have 50% less chance developing dementia. But, how can you drink 6 cups of coffee per day if you do not have good health? It can be that the "small positive effects of caffeine" are due to caffeine consumers have a little better health overall and these with worse health have to consume less caffeine.

For an anecdote, I once went completely off caffeine for a month and haven't noticed a thing after first three days, when I was unusually sleepy. Even training results followed the same progression.

Exactly. I mixed soda syrups for my small business and had to learn what actually goes into it all. Soda is generally 80% soda water, and the syrup is usually only 50 °Bx (roughly 50% sugar by mass). Of course with sweetners the syrup is almost entirely water since you need a tiny amount of the sweetner to get the same level of sweetness. In an average soda drink, the flavor and dye is about 0.6% by volume of the syrup (again, more water since it's in solution), and the preservative is 0.2%. The amounts are so tiny you would have to consume a completely unreasonable amount to even get sick, let alone do lasting harm (except for the sugar, obviously).