Leaving Android, to me, seems like a bad idea for a 3rd party at this point. FireOS can already run Android apps which is a massive gain over launching another OS and having to support foreign apps via emulation or push the development/porting of apps. What's the gain to developers if you're the smallest player in an almost saturated market?

Additionally, Amazon makes a lot of money off of services and digital content, not just physical goods. Pushing (like with the Fire HD tablets) integration with those into the base experience plus running their own Android app market (they can always go under Google Play's percentage, and if they gain market share they'd get a lot more apps loaded to it) seems like a more lucrative and compelling option.