I think the point you raise is related to the fact that caffeine is a diuretic, which means it makes your kidneys clear more fluids than they normally would and you end up making more trips to the restroom. I doubt that this is a problem if you are drinking in moderation.
The bigger concern I would pose is about the nutritional qualities of soft drinks. I don’t have any bottles handy since I don’t drink them but I was unaware that they contain electrolytes. In fact, I thought they did not contain any electrolytes. I’d be curious as to what they have. As for the carbs, the carbs in soft drinks are low quality carbs from processed sugars. Not a whole lot of value in those honestly. All of this said, again, I don’t think this is a big deal if you are drinking in moderation.
In the end, I would say that drinking soft drinks in moderation is not going to do any kind of great harm to your running. Now, if you’re drinking like I see some people around me drinking, I would suggest at least cutting back.