Your argument suggests that there are actually 2 smart phone markets. One for iPhones, one for everything else. I think this is a fairly reasonable assumption for the majority of consumers.

Let's assume very few people are switching ecosystems at this point based on form factor. That would mean Apple made a new product to cannibalize 5% of their existing market. No similar product exists in the android ecosystem. It seems reasonable that an android phone maker could get similar market share but have these sales come from a combination of their existing sales and competitors sales.