Olive Oil Times, formerly a site with a good reputation, ran an article with the heading 'Diet Soda Linked to Weight Gain, Not Weight Loss'.
Evidence from RCTs does not clearly support the intended benefits of nonnutritive sweeteners for weight management, and observational data suggest that routine intake of nonnutritive sweeteners may be associated with increased BMI and cardiometabolic risk'.
Well, that was a strange outcome, because a previous study that used the same data reached a different conclusion 'Overall, the balance of evidence indicates that use of LES (Low Energy Sugars) in place of sugar, in children and adults, leads to reduced EI (Energy Intake) and BW (body weight), and possibly also when compared with water.
What I think is that the effects of all interventions to combat obesity are limited and inconsistent. There are many variables and no study will ever be able to control for all of them. People might drink diet soda, but still eat to much fast food, nullifying the effect of the zero calories of the diet soda.
So, if you are trying to lose weight replacing sugary drinks with low calorie drinks can be a helpful part of your overall strategy. It will not be a panacea or make weight loss easy. See here.
The Olive Oil Times made thing even worse by asking a naturopath for her opinion. 'Carolyn Dean, medical doctor and naturopath, didn’t mince words in giving her opinion about the research. “This study, which exposes the false claims of synthetic sweeteners, should have the industry quaking in its boots”'.
As Wikipedia rightly warns: 'Naturopathy or naturopathic medicine is a form of pseudoscientific, alternative medicine.' Poor Olive Oil Times.
 Azad et al: Nonnutritive sweeteners and cardiometabolic health: a systematic review and meta-analysis of randomized controlled trials and prospective cohort studies in Canadian Medical Association Journal – 2017
 Rogers et al: Does low-energy sweetener consumption affect energy intake and body weight? A systematic review, including meta-analyses, of the evidence from human and animal studies in International Journal of Obesity – 2015