I found this article, suggesting we would be making it difficult for ourselves if we decommissioned the on-prem Exchange server. Doing so makes it difficult to manage users with mailboxes and mail objects such as distribution groups.
blogs.technet.com/.../decommissioning-your-exchange-2010-servers-in-a-hybrid-deployment.aspx.
It looks like we need to keep an on-prem CAS Exchange server for management.