Hangouts work, but it's heavy and not a good experience overall. And facetime is ios only, so Duo will definitely bridge this gap.
I read on a Verge's article that Google wants to make Hangouts enterprise focussed by integrating more into the Google apps and keep Allo/Duo as the consumer focused communication apps which i think is a great strategy.
Once all of android users get this pre installed, i think its not difficult to get rest of the users use this.