Unfortunately your example isn't a full reproduction as both the Ember/React examples reuse existing DOM which is important for selection state and the popover functionality.
One of the best articles I've seen written about the future of the web and moving past the constraints developers place on UX due to the way we've built web apps in the past.
Actually, it doesn't "force" you to use get/set. You can just POJOs just fine w/ Ember, but you won't get property observation or unknownProperty support.
I also question your assumption that Object.observe will be as fast as Ember's observation system. I wouldn't be surprised if it was several times slower.
Object.observe also doesn't support array observation, which is something we support in Ember.