If the diabetes is under control with insulin, food, etc., cats can live a normal, happy lifespan. This does not have to be a death sentence for a cat. Many diabetic cats that have died, died from other issues/complications that had nothing to do with diabetes - such as kidney failure, cancer, etc.
There is no reason to think that if treated for diabetes and in remission, diet controlled or on insulin a cat can live whatever the normal course of its life is and hopefully die from being old.
It's the same as people. If a diabetic person is treating their diabetes and taking good care of themselves, there is no reason to think that person won't live a long normal lifetime.
Hope that helps you.