It's interesting because as far as I can tell, Fiber matters more in environments like ReactNative where the JS runtime talks asynchronously to the underlying native rendering engine. The implications are not as significant on the web. On the other hand, the current Fiber implementation is just "laying the groundwork for further experiments" - I'm also curious to see what concrete use cases can be built on top of it, before that, we don't see a strong need to implement something similar.
Also, technically, Vue's update system already processes component updates as discrete jobs in a scheduler, so instead of a complete rewrite we may be able to improve the current system to reap some of the benefits that surface from Fiber in the future.