A study of human type 2 diabetes patients showed that restricting carbohydrates and iron in combination with polyphenol (polyphenols extracted from tea) enrichment was nearly 50% more effective than protein restriction in improving renal function and increasing survival [DIABETES; Facchini,FS; 52(5):1204-1209 (2003)].
It might be one of those areas where it's possible to find supporting evidence for many different causes, treatments and outcomes, depending on what it is you'd like to hear!
I know from my local PCT that their type 1 diabetics typically have one third of their original kidney function remaining 25 years post diagnosis. It seems far more likely that the damage is a result of glycation reactions resulting from elevated blood sugar levels than a general tendency to eat too much protein?
My own experience is that, since I began eliminating carbs from my diet 8 years ago (and included more protein), my kidney function has steadily improved.
There may be a level of kidney function below which the damage is irreversible and protein needs to be restricted, but I suspect it isn't the protein that causes the initial damage.
All the best,
fergus