Thus, in 2 years; one way or another Apple will be available in both markets (AT&T and Verizon - and T-Mobile, ect). The cost of entry will be high - $199 for the iPhone 5 if histrorical prices remain consistent. IMHO, a great many Verizon users will hop on the Apple platform due to the phone's superiority, but the price will impede a certain number.
However, in year 3, the iPhone 6 will be out - and the $199 price for the iPhone 6 will mean that the iPhone 5 will drop to $99. This lower cost, and the easy availablilty of apps from iTunes (plus whatever other goodies Apple continues to add to the iPhone) will make the iOS more desireable.
Gartner's approximateion of 14.9% seems surprisingly pessimistic - given that the PRESENT marketshare for Apple is 28%
You bring up logical points but I’d imagine Gartner considered all of those factors. They usually do.