One could have said more or less the same about Symbian not that long ago.
> Nobody defines Android in this way, by a single component, and in a way the author even acknowledges this.
That's right, it's not defined by any particular software component. Any particular component can be replaced until the whole thing is replaced, a la Theseus's ship. So what is Android - what can't be replaced?
The answer is the development model. This can't be replaced because it's definitional to Android. You said it yourself: Android is "an open source operating system." More specifically, it's an open source OS that follows a particular development model:
1. Development of mainline Android happens behind Google's closed doors, and is thrown over the wall to OEMs. 2. Development of Android for particular devices happens behind OEMs closed doors, and is thrown over the wall to customers.
#2 is the rub. It annoys customers, who have to wait to get updates and fixes. It annoys developers due to the fragmentation problem. And it annoys Google because they don't control the update cycle, and instead have to shoehorn things into Google Play Services, etc. But critically it doesn't annoy OEMs, who love being able to apply their kernel "fixes," shovelware, and other familiar Android condiments.
Google would love love love to transition to a ChromeOS-like model, where they can exercise a lot more control. But they can't just do that outright - OEMs would revolt. They probably can't do it at all within the confines of Android and the development model they've established. That's their key problem.
> But why would anybody care, especially if its transparent to users and even to developers
It won't be. There is no universe where Google can provide a new OS that works on all their customers devices and supports all the apps. The best they can hope for is a new OS with a compatibility layer that works well enough - think Apple's transition from classic Mac OS to OS X.
But as soon as OEMs get wind of those, you can bet they'll be tripling-down on Tizen or forks or whatever. That's the dilemma that Google finds itself in.
Android is a sort of asymptotic dead-end, where Google can keep providing fixes like Dalvik->ART, but those can't address the essential problems. Eventually those problems will be severe enough to open a space for a new offering to thrive. Google hopes to own that offering, cannibalizing Android instead of letting someone else eat it.
(Oh, but then there's China...)