They can. But depending on how you used Ansible before, it may mean a heavy rewrite of your deployment strategy. I'm not saying it will always take that long. But depending on your app, the requirements may be very complex and not fit into the docker idea.
(it always means some extra work for security updates though - now you're updating both the host and images)
It's not. It would be months of work if they wanted to convert all their Ansible code to Docker, but that's by no means required.
Docker and Ansible can easily coexist peacefully.