>The basis of Android is open source ...
Nominally, yes, Android is open source, but actually Google has a monopoly* on it akin to Microsoft's on PC OS: Google has perverted the openness of the system by creating/moving APIs that many apps depend on into proprietary components outside of the AOSP proper, moreover, Google has the only viable app store for Android.
Microsoft's becoming monopoly on PC operating systems depended on the "network effect" where a product or service becomes more valuable the more users it has, which happens because of the closed feedback loop: more/better software available for Windows -> more Windows users -> writing Windows software becomes a better proposition for developers (than writing for competing platforms) -> more/better software available for Windows ... This soon results in a natural monopoly which is practically impossible for a competitor to challenge.
With Android Google not only has an OS monopoly akin to Microsoft's as such, but this is reinforced by its app store monopoly (Microsoft is working on the latter for Windows as well, but not quite there yet). Google's paying for Android and the related services from its advertising revenue makes it even more infeasible for a competitor to succeed and therefore one is unlikely to even emerge: for Google Android is good business as it helps them to rake in advertising revenue, for a potential competitor not so much as it would have to sell the OS as such or embed the cost into price a physical product thereby making those more expensive which means they cannot really compete (as Android is "free"**).
* technically, an overwhelmingly controlling market position, colloquially a monopoly (which is close enough as it is equivalent to an absolute monopoly for most intents and purposes)
** not really free, of course: 1) the consumers ultimately pay for it in higher costs of advertised products and services (only they pay more as there are middlemen) 2) they also pay with their information / loss of privacy and 3) with having to put up with commercial propaganda (=advertising) i.e. getting actively manipulated and misled, which 4) is a systemic problem in a market economy depending on well-informed parties to work efficiently (hence, again, resulting in actual monetary cost because of less efficient economy due to a group of market parties actively working on consumers to be less than well-informed in their decisions)