I like this idea, but I think 2019 might be ambitious.
There are already many good but abandoned or unsupported mobile OSes available: Firefox OS, Ubuntu Touch, Sailfish OS (this with the N9 Swipe UI would be nice!), Plasma Mobile as well. Blackberry 10 was a great OS but is almost abandoned now... and Windows 10 Mobile IS a good mobile OS, but we've already been there!
Samsung isn't getting anywhere with Tizen and they are by far the largest producer of smartphones in the world. If Samsung can't do it, can Nokia? Does Nokia really need to take the risk of annoying Google (this would surely kill the deal for quick Android updates and long term support)?