“As long as you don’t” is doing a lot of work there. Humans regulate their eating using hunger and satiety. If you eat a diet that calibrates those well, staying at a good weight is massively easier.

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.