Studies based on the self-reporting of lifetime consumption of a particular food are always questionable, but for me the main problem is that there are a lot of different sweeteners and presumably they have different physiological effects. Looking at the full study on Sci-Hub, the researchers don't seem to have explored this at all.
Also the data for dementia and for the risk of stroke in general aren't really that compelling, but the numbers for ischemic stroke specifically are impressive; almost all P values < 0.01.
In one book about cancer prevention and experimental oncology from the 80's it is written, that
"the new compounds and substances, that invade our life appear so fast, that we" - he probabbly means the experimental oncolgysts - "can't check their biological effect on the human cells not mentioning providing studies for long term usage .. ".
This has been the situation in the 80's 
Now I suppose, that it is worse.
We are being constantly bombarded with new molecules from everywhere - colorers, sweeteners, new compounds for plascics, softeners, desinfection ingredients, cosmetic substances, conservants, whatever you can imagine. And everything this we eat, drink, touch, even breath every day without even knowing that it can cause many negative side effects for the health, including cancer.
That most probabbly is true, and I have seen it in my practice in the form of literary desolved enamel in regular coca cola drinkers.
That is one of the reasons, that I avoid eating american diet and mostly cola and hamburgers. I also never go to McDonalds and KFC. Also I try to avoid at some extent artifitail substances, but it is practically impossible.
