Android is open-source in this same way id Tech is 
     open-source

On the stuff released by id -- that's pretty awesome what they are doing there and a lot of enthusiasts and even small companies have built stuff on top.

Of course, Android is much more than a game engine and the difference between Google and id is that being open-source comes in Android's value proposition.

    it's no longer commercially viable

The Quake engines are still commercially viable for mobile phones. It also beats starting from scratch when low on budget.