Well that's another discussion to be had because when was "diabetes" actually classed as a condition?
Just doing the Goole seem the Egyptians circ 1500BC were classing it as "too great emptying the urine" which is believed the first reported cases so its been around a long time, even longer than you
I don't know, but don't forget that in the past what we would now a days class as children would certainly had kids and so passed on many genes, which are remarkable resilient.
True Banting has no doubt saved many many lives and allowed us to last longer and reproduce at a prolific rate and so making sure that the defect gene and many other still stays in the population.