OK so why not Google Chrome web browser on top of Fuchsia, instead of Linux kernel and the usual user space stuff? Google can call that retrofit anything they want, so it could still be Chrome OS.
Besides this of course, Fuchsia shared many other components with chrome (the use of Skia for example). If we look at the direction for Flutter, Fuchsia and Chrome, it is clear that this is not some casual side-show but a very well thought out strategy.
It is although true that Google could replace the low layers of Chrome OS as long as it replicates most of the user facing features.
If Google intends to make Fuchsia some kind of Android successor (we have no way to know if that's the plan though), this feature is utterly needed anyway : an Android VM for 'legacy' apps and Flutter for the future.