I recently used a 35+-year-old tube whose tire tread had completely cracked and flaked off of the tire casing. I'm pretty sure that tube had been sitting in that tire for the tire's whole lifetime, and the tube was perfect.

I remember reading (Sheldon I think?) that if a tube is held in its "natural" shape inside a tire, it will be very unlikely to degrade at all, but if it is folded into a box, it will be prone to cracking at the folds.

I think that the self-gluing phenomenon is from extremely mild vulcanization due to heat in storage and on the road, not due to degradation of the butyl.