I'm relatively ambivalent to a walled garden if and only if there's a (reliable) way to opt out of it. In Android's case, it's not only quite possible to opt out of Google's garden, but arguably rather commonplace (Amazon's "Fire" devices, most custom ROMs by default, etc.). In iOS' case, it's outright impossible.

Opting out will be harder and harder because technologies like SafetyNet will make most apps unusable on non google approved devices.

Android with google services replacement MicroG GmsCore runs most apps without hassles.

which makes me wonder why amazon or facebook haven't started leading an open-source alternative to google play services.

It's such an easy way to run the mobile race compared to start ing from scratch.