If 90% of iOS users upgrade two years after their device purchase then next year will result in 90 million iOS devices sold to existing users.
No other phone manufacturer has such high customer loyalty and satisfaction.
With currently 250 million iOS devices sold so far the 2 estimates place Apple at selling 225 & 280 million iOS devices in 2012.
Put it this way. There is a strong likelihood that there will be over twice the number of iOS devices sold by the end of next year than there have been sold thus far.
Couple that with a strong profit margin on their devices and I have no doubt Apple will be the biggest tech company for a while.