From what I've seen, I believe that Fuchsia is intended to eventually replace GNU Linux based Android as the mobile platform for Google... eventually. The idea is to have a good platform for VR/AR apps (soft real-time processing / rendering), while providing a system that can be continually upgraded.

Unlike the current situation with the Android ecosystem right now, where a newly released product is lucky to get even a few security updates in a year or so, and have effectively had support dropped after that.

Compare that to the ChromeOS ecosystem, where Google guarantees 5 years of support from the time the first production on a platform was released.