It turns out that the only retention policy that will be automatically applied is one named "Default MRM Policy". That name is hard-coded into the product, so if you delete it all you need to do is recreate it with that same name to make it the default policy. There is no property for that policy that makes it the default. It's just the name.
Here's some of the back story. Exchange 2007 introduced Managed Folders, which was an early form of retention management, but it was rather clumsy to work with. Exchange 2010 SP1 and later uses message records management (MRM) policies and policy tags. MRM policies are collections of policy tags that tell Exchange server how manage mail data. Some tags apply by default, such as the "Default 2 year move to archive" tag. Others are personal tags that users can chose to apply to one or more mail items, or entire folders, such as "1 Year Delete". Exchange setup creates the "Default MRM Policy" which includes a default set of policy tags.
The Default MRM Policy does not actually apply to any mailboxes by default, unless the the mailbox is given an archive mailbox. At that time, Exchange looks for an MRM policy named, "Default MRM Policy" and automatically applies it to the mailbox. If the Default MRM Policy does not exist, no retention policy is applied.
Set-Mailbox <username> -RetentionPolicy "Sales Dept MRM Policy"For hybrid customers and those using Exchange Online Archiving, you manage MRM policies for on-prem mailboxes on-prem. The policy settings for these mailboxes will sync to Exchange Online with the DirSync process and will be applied to the online archive mailbox.
If a user mailbox and archive mailbox are both in the cloud, you will apply the cloud version of the Default MRM Policy in Office 365. Here, you should configure the Office 365 version of the Default MRM Policy to match the same settings as on-prem.