The main risk of sugar isn’t the calories themselves, but rather their effect on our fullness perception. That is, the more sugar we eat, the harder it is to feel full after eating something. This in return cases a vicious cycle, one that can easily lead into obesity. I don’t know if that same issue can happen with sweeteners but I don’t generally trust anything that tricks our senses to such a degree. I don’t consider coffee the holy grail either, it’s just that its negative health effects have been tested for ages and are acceptable for its overall benefits. But that’s my own risk assessment, with only my health in the line.
It’s hard to get a good grip on the health neutrality of diet soda when the companies who make them have lied to us about sugar for decades. Maybe sweeteners are just their next lie, who knows. Much of the research done on sweeteners is funded by the ones who profit from it. The food industry have far more power than anyone should be comfortable with them having.