The insulin injections also allow diabetics to regulate their blood sugar so that they don't have to deal with even bigger problems.
I have spent a lot of time around 4 diabetics in my family, none of which had morbidly unhealthy lifestyles. Sure, they could all use a little more exercise and moderation in their diet, but then again most people can.
If you spend a lot of time around diabetics you'll met many people who have lost appendages and continue to live an morbidly unhealthy lifestyle.