If you truly believe that these will net-dehydrate you, it should be pretty easy to prove - just survive for 1 week drinking only soda/juice/coffee. If you die of dehydration, we'll admit you were right :)
I've survived for years drinking mostly iced tea, with a soda in the morning instead of coffee. I can count the number of times I drink plain water in a week on one hand (after exercise, basically)
I can confidently admit that when I was a stupid teenager, I had consisted on Soda & Coffee and potentially only getting raw water content absorbed from foods for multiple weeks on end.