I certainly don't want to pretend to be an expert when (1) it's my wife, not I, that has diabetes, and (2) she's only been diagnosed for less than 2 months. But being a pretty big nerd, I've been reading from a textbook about diabetes, and one of the things it mentions in a chapter about causes of diabetic complications is that in experiments with diabetic dogs, the dogs spent 2-1/2 years with unhealthy hyperglycemia. But during this time, the dogs generally did not suffer diabetic complications. The dogs then spent 2-1/2 years with very good blood sugar control. However, by the end of that second period, many of those dogs developed diabetic complications. The takeaway from the experiment was that hyperglycemia can damage the body in ways that don't really manifest themselves in visible, detectable ways, but that damage could later cause debilitating and dangerous diabetic complications long after a diabetic has established good blood sugar control. (The reason mentioned in the textbook is that hyperglycemia can damage mitochondrial DNA by causing the mitochondria to produce a damaging molecule called superoxide and possibly cause mutations that can mess up mitochondria---the cells energy plants, so to speak---in ways that would make someone more susceptible to diabetic complications later, even after getting BG under control.)
Regardless of the extent that anyone should worry about a particular cause of later-in-life complications from earlier-in-life lack of control, it shouldn't be surprising that pretty much anything that is unhealthy, be it something that is chosen like smoking or excessive drinking or something that isn't chosen like diabetes, can damage the body in ways unseen and lead to problems many years later, even when one gets whatever problem it is under control.
The body is remarkably resilient though, so it's also not surprising that many people's bodies seem to forgive them for their earlier chosen or unchosen unhealthiness. But even as a non-diabetic, I try to take my diet seriously, realizing that what I do now will affect my health down the road.
