Assuming Google is even targeting the developer ecosystem here. I'd assume that most of the users of Fuchsia will be end users, so that's what Google is targeting.
How much of the Android app ecosystem has a strong dependence on POSIX compatibility? I mean, yes, it is Linux under the hood, though it's not glibc, but most software is written against the SDK which abstracts over all of that.
Not being POSIX doesn't mean it's going to be entirely foreign or that it will have no compatibility with software that is already written for POSIX-like platforms. It just means that the basic process management, IPC mechanisms, permissions handling, and so on uses a model which is akin to a cleaned-up subset of POSIX; everything is done via handles, which are a lot like file descriptors but work in a somewhat more uniform way, there is cleaned up management of resource allocations to jobs, memory mapping of processes, and thread management, and so on.
It's not terribly hard to build a POSIX-like layer on top of this; said layer isn't necessarily going to support some of the real warts of POSIX, like the really broken way signals work in POSIX, so some software that intimately depends on this may have to be factored out to support the way that Fuscia handles signals, but for most software this will be a big improvement. Software on POSIX systems nowadays has to jump through hoops to make signals play well with event loops, frequently allocating a pipe that gets written to in a signal handler, so the event loop can pick up that notification later, while in Fuscia that's how signals already work, it's just a state change on a handle that can be waited for in Fuscia's equivalent of select/poll. So in cases like this, Fuscia will add a new code path that is simpler and more maintainable than the one on POSIX-like systems.
