A single study show that there is a link between diet soft drinks and Diabetes.
(http://care.diabetesjournals.org/content/32/4/688.full)
One explanation offered for this is a rather convoluted one that the gut bacteria is mutated by the soft drinks using artificial sweetener, aspartame as its means into rejecting sugar. Hence causing diabetes.
While I think the link is correct, the user or abusers of diet soft drinks do tend to get type 2 diabetes, the cause is far simpler then that.
When you drink a diet soda you think you are hydrating yourself as it is liquid.
In fact, diet drinks tend to dehydrate you. It is a diuretic, which means you need the bathroom more causing a net loss in water.
Studies show that people think that as it is “Diet”” you can take more and more because it is healthy. (This happens to all foods and drink labeled diet.)
(http://www.dailymail.co.uk/health/article-3137553/How-called-healthy-snacks-harm-good-People-eat-likely-overindulge-avoid-exercise.html)
So when you drink many diet drinks, you dehydrate yourself. And over the long term of excess use of diet drinks you create inside of yourself a long term state of dehydration.
Last link, dehydration causes an increase in blood sugar. Long term increases in blood sugar lead to Diabetes
Conclusion, long term dehydration cause damage to the body such as… Type 2 Diabetes. There Diet drinks as the cause of dehydration can help cause Type 2 Diabetes even if there is no direct link top diet soda, but a link to the life style around it.