Diabetes has been cured many times, just not in humans. Researchers have managed to come up with various ways to cure mice. However, none of these have proved to work with our species. I believe that a cure is possible. I also believe that I won't see one in my lifetime. To develop a cure in a short timeframe, researchers would need unfettered access to human test subjects, and few or no restrictions on experimentation. This would result in many human rights violations and untold suffering, and even then there would be no guarantee of success.
However, think about where we are now. 100 years ago, all of us would be either dead or waiting to die because of Type 1. 60 years ago, we would have had no ability to test our own blood sugars to manage our insulin doses. 30 years ago (when I was diagnosed), insulin pens weren't a thing (or if they were, they were not widely available). I still remember my early childhood, with a long syringe needle that my mum had to clip before putting it into the sharps bin. My blood testing machine took two minutes to deliver a result. 15 years ago it took 5 seconds. A year ago I got my first Freestyle Libre sensor, with results delivered in 1 second.
Injected insulin itself has gone through amazing transformations. It wasn't until about 18 years ago that I went onto a basal bolus regime, which has allowed me to match my insulin and food intake more accurately. There is also more choice than ever regarding how to administer insulin (syringes/pens/pumps); new insulin is coming onto the market regularly (e.g. Fiasp). Long-acting insulin has undergone innovation, with Tresiba having a longer effect than Lantus or Levemir.
If you're waiting for a cure, holding your breath will result in suffocation. But with new ways of treating diabetes appearing so often, managing our condition will continue to get easier.