[nate_meurer]

High blood sugar is toxic to pancreatic beta cells. Thus, poorly controlled type 2 diabetes typically progress to some degree of insulin-dependent diabetes. That condition is the worst of both worlds; your insulin levels are insufficient, but even supplementing insulin artificially can't bring your blood sugar under control without significant lifestyle changes.

Note that beta cells are hardly unique in this regard. High blood sugar is toxic to all tissues, but the affects on some tissues are especially problematic, examples being retinas, kidneys, endothelium, and peripheral nerves.

[RHSeeger]

The reverse tends to also be true. After being a Type 1 diabetic for a good while, it's not uncommon to also start showing signs of insulin resistance (type 2).