Mail retention - storage of potential duplicate Data

Thanks for you Answer. But my question is more about the Mailboxsize itself, rather then deleting items.

We force our users to sort their mails into PublicFolders or Groups. We do this by a strickt limited Mailboxsize. Mail retention (as required by law) is done by Journaling right now. 

If we move to the retention policies in Microsoft Purview and all the items stay in the Mailboxes, even if they are moved to PublicFolders or M365 Groups, the Mailbox will grow and grow, until the retention policy allows deleting (this is after 10 years for some mails). 

Also, an associated online archive is actually a second mailbox, archiving would not free up any space in the primary mailbox. This would mean, that the Microsoft Purview retention policies are not a equal replacement for Journaling Mailboxes.