I doubt the OS itself will be a bottleneck. All of Apple's devices from phones to computers pretty much run the same kernel. A lot more devices from supercomputers to toasters run Linux. I'd say the OS is already here.
I had high hopes for Apple's Marzipan toolkit, and time will tell if it becomes what i hope it will, but for now it mostly seems to be a way for iOS developers to bring dumbed down applications to the Mac.
And ultimately we'll all end up with a bunch of Electron apps doing almost what we want, only slower.
And people certainly thought about it. Phone manufacturers just actively work against people running alternative OSes on their phones, by locking up their devices, and making it annoying to unlock them, and by not respecting GPL, and releasing code with huge delays and in a very halfassed manner, and by ignoring calls to upstream their code.
I believe all mainstream OS's we have today, coming from the pre-mobile era are simply not suitable for this task. If it was possible at least Apple would have built a docakble computer already, and I'm sure they have considered it but probably realized their OS will be the bottleneck. It's already too heavy, there's too much legacy, and a complete rewrite would be an insurmountable task for them.
This problem requires a new, much more efficient and compact kernel/OS that will be free from the legacy of the pre-mobile era. Something designed specifically for low-power mobile devices, possibly with conceptually new kind of a file system with (I actually have some ideas what it could be).
Anyway, just some vague intuitive insights. This is an interesting problem, a niche that's opening in front of us right now, just begging to be filled with something new.