I don't know about overtake, but GCE is IMHO far superior to EC2 in terms of both performance (both hardware and network) and price right now. And they keep dropping the price by 30% pretty much every year. I expect to see a shift to GCP over time, yes.
Re Citus:
Considering that you get 2 GB of bandwidth per core on GCE, that makes a lot of sense for a distributed DB like Citus where there's a lot of inter-node traffic.
That other point there is that the maximum amount of RAM for a VM is 208Gb, which is not enough to scale up a Postgres instance. So a barrier to entry is porting your entire data layer to Datastore -- that's a significant engineering challenge that most people are unlikely to take up.
So Citus would fit in really well. IMHO focusing on AWS (where Amazon are providing their own DB services) is missing an opportunity on GCP (where Cloud SQL is MySQL based and fairly limited).
That kind of growth would make the growth of AWS look like a local corner-store.