You can only get adoption of a non-POSIX OS if it offers something really unique to the user, or is in a very different market space.
You should encourage people to make native software using the new paradigms and abstractions of the OS, built from the ground up with decades of hindsight, rather than encouraging crappy ports.
> You can only get adoption of a non-POSIX OS if it offers something really unique to the user
At this point, just not being built on 50 year old crappy standards would be pretty unique. I have my own personal wish-list of features that would lure me in even if the only hardware supported was an RPi.
I'd like to see other paradigms explored, too, but it seems unfortunately hard to get there from here.
One thing is for certain: you will not replace the current players by cloning them.
That said, I can't imagine Linux actually having gone very far if it wasn't a UNIX clone. I think it would have been a real long uphill battle and it would have gotten handily beat by whatever open source UNIX clone came along next, for the pure reason that it's much easier to deal with POSIX compliance (and all the software it brings that actually makes it worthwhile to run the OS) when trying to copy a system that already did it and worked out the kinks.
I mean, if someone came out with a free (maybe open source) version of Windows at that time which was legal, we could easily be living in a world where that's the dominant free OS.
I guess what I'm saying that that being vastly better on one metric (in this case, free) doesn't necessarily mean you'll take over a market (because a free operating system that can't run any software isn't very useful), and just being a clone won't either, but being a clone that's generally as good one most metrics and vastly better on a few others (e.g. a UNIX which is also free and open source, instead of just some other UNIX) may.
That said, there is a lot of room to innovate in OS/userspace boundaries that don't rely on POSIX APIs--see epoll/kqueue/iocp for one place where this innovation already occurred. I'd love to see better alternatives to POSIX signals, for example. So an OS that treats POSIX as a mere compatibility layer and not the primary driver of its design could be welcome.
Then who cares about the OS? Why even bother. Just use ChromeOS.
But the really fun possibility is having an OS where "Web" and "Posix" are equally-placed personalities that the OS ran, not only preventing rogue web pages from touching the posix system, but also preventing rogue posix processes from touching the browser (except where the end-user intentionally joins them through the system-provided Open File dialog or by opening the case and removing the debug pin to unlock cross-VM debugging). ChromeOS sort of implements that today, by running custom Linux environments in containers, but I'd rather trust Xen than Docker as the underlying isolation layer.
Do not attempt to make a new posix system, because we already have giant, entrenched, mature posix OSs and you'll bring nothing new or interesting enough to the table to warrant switching while being held back by the creaking old standard.
I'd be willing to make a lot of sacrifices to move to an OS that met my personal list of wants.