Not necessarily. If Apple allowed third-party app stores, alternative browser engines, had better cross-platform messaging support, et cetera, a lot of Android owners would buy iPhones.
A significant reason why Android appeals to many folks is that it represents a more open alternative to the iPhone. By opening up their walled garden, Apple still stands to benefit by magically becoming more appealing to a big chunk of Android owners.
I unfortunately think you're dramatically overstating how many people would actually switch after they're forced to open up rather than finding some other goalpost or simply not caring anyways.
A significant reason why Android appeals to many folks is that it represents a more open alternative to the iPhone. By opening up their walled garden, Apple still stands to benefit by magically becoming more appealing to a big chunk of Android owners.