1. correlation is not causation. Many (most?) people who use artificial sweeteners already have poor habits with food and exercise, so it makes sense that their outcomes would be worse. In fact using AS may provide people with an excuse to continue other unhealthy habits.
2. The assumption here is that the only reason for using AS is to try to lose weight. For me it is about consuming less sugar. The science is unsettled on this, but my default position is that managing sugar intake (including simple carbs that quickly and easily become sugars when eaten) is the smart approach to diet.
Many people want to confirm their almost dogmatic assumption that AS are unhealthy. This is another example of the naturalistic fallacy.
Few things are more discouraging to someone on a weight-loss plan than the oft-cited statistic that 95% of people who lose weight will regain it within a few years. The difficulty in sticking with a long-term weight-maintenance plan is one of the main reasons that weight-loss programs fail. To uncover clues to successful weight loss, researchers have been collecting information on people who have lost weight and successfully kept it off for many years. This project, known as the National Weight Control Registry (NWCR), records what these people did to achieve their goals.