As a user, no way I would just toss aside years of app purchases and past experiences on one platform to suddenly decide to switch. Sure, some people do but generally they don’t. Switching cost applies to lots of things, not just phones, yet the inability of app stores to transfer proof of purchase makes this cost higher than it would normally be.
As a developer, like it or not you will depend on some (usually large) percentage of APIs available only on your first chosen platform. Probably even the programming language! And for the most unfortunate of businesses, a ton of this effort will be spent up front before even knowing if some new idea for an app will be denied entry to the App Store due to some unwritten rule (or worse, denied a few years in as the rules shift). If you reach the conclusion that you picked the wrong first platform, most developers are not “free” to “just” pick another phone/platform and make it all work out before going broke.