my apologies there is no perfect BMI and it is age dependent along with many other factors.
however when you say "Diabetes is not caused by being overweight, contrary to popular myth. Both are caused by the same root reason, an unstable blood sugar level and overall metabolism caused by a diet way too high in processed carbohydrates" is the bodies matabolism not slowed by lack of exercise and healthy diet ?, and over weight people are often (NOT ALWAYS) due to consuming too much of the above (which you say are the main factors in contracting diabetes).
Personally if i get to a size which makes walking up a small set of stairs tiring and you have to buy a seat for each bum cheek on a planeexcess i'll be walking up a few more stairs and seeing a diet consultent.
However i think Tim is a happy medium and we all have our own opinions, mine is that if you want to loose weight and you are able to do so then just do it and i'm sorry if i have anniod you with this it is a touchy subject for many people.
on a lighter note and keeping with the thread subject Odds of being killed by freezing - 1 in 3 million.