Ok evidence based to start with, I don't know how else we can really evaluate .
As far as I know , there are no trials that will show the long term effects of people diagnosed with diabetes keeping levels below 5.6% There are trials demonstrating that for people with both T2 and T1 diabetes it is beneficial to keep levels below 7%. There are also studies that show that driving these levels down with some drugs might not be beneficial (I quoted,linked but then cut the studies, I have them if anyone wants)
Fifty years though is the question and I think we really don't know.
The definitions of what is and isn't diabetes has changed since then. It isn't so long ago that diabetes wasn't diagnosed until fasting levels reached 140mg/dl (7.7mmol/l)
Fifty years ago T2 diabetes was only recently defined, it was called maturity onset diabetes, no-one would have thought to test for it in a young person. It wasn't diagnosed until symptoms appeared ie weight loss, kidney problems or neuropathy, by that time the person had severe beta cell loss. I also think there were probably people who had T2 and were, because they were young , diagnosed as T1. They would have been put onto insulin which would have worked.
There are most definitely people who were diagnosed with T1 fifty years ago and have now been found to have MODY (miss named because they didn't understand it). Some have later been rediagnosed and sucessfully transferred from insulin to oral medications. I also (and beware anecdote) know of a man on one forum diagnosed in the 1970s as a T2, put straight onto insulin with a daughter diagnosed not long later as T1. He still considers himself to be T2, both are doing well.
Lastly, and again anecdote , I have read on forums of several people diagnosed in their fifties and living until they were approaching 100.
My conclusion is that, as with T1 and MODY, we all face similar problems caused by high glucose levels, all we can do is be positive about our futures. I know I am.
There is a book concerning this 50 Secrets of the Longest Living People with Diabetes by Sheri Colberg(she has T1 but researches exercise in T2) I haven't read it but have heard it praised by people with oth T1 and T2.