From a comScore press release issued April 19:
Before the release of the iPhone 7 models in September, the iPhone 6 device family had continued to rapidly expand its footprint. According to comScore MobiLens®, there are still nearly 50 million iPhone 6 users, in addition to the more than 20 million iPhone owners who have a 5 model or earlier. These 70 million collectively represent a huge base of users who are ripe for an upgrade cycle later this year when the iPhone 8 is expected to be released.
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The ripeness of Apple installed base is one reason Wall Street is so excited. See Apple analysts scramble.