Any OS w/o a POSIX interface is probably a non-starter at this point. Anything POSIX is arguably UNIX. This said, who cares about the kernel? Android is not about the kernel. There are 100x more difficult problems up the stack than the kernel. My guess nothing will happen there. It has been too hard to get Android where it is.
My pet theory is that Google intends to eventually port the existing userspace for chromeos and Android to the Fuschia kernel.
As long as your apps run inside Chrome's runtime, or interact with the Android APIs instead of the low level POSIX APIs, they'll never know the difference. (And we all know that Android's existing POSIX support is minimalistic, to say the least.)
In a twisted way, POSIX is C's runtime that the committee wasn't willing to make part of ISO C.
So unless one is coding in straight C there is very little pressure to care about POSIX anyway, as all other languages, including C++, have a better OS independent standard library.
https://www.reddit.com/r/Fuchsia/comments/7pywqu/how_unixlik...
https://www.infoq.com/news/2018/04/google-fuchsia-zircon-ear...
https://fuchsia.googlesource.com/zircon/+/HEAD/docs/concepts...