Timothy,
You have a few options to minimize your risk of losing data and/or operating in a less secure environment when users leave your organization.
The most common scenario I come across in enterprise environments is the following hierarchy:
1. Have a secure Geodatabase administrator, the SDE user.
2. Have as few data owner schemas as is required, that are centrally managed and that the DBA or database team is aware of.
3. Create/enable user accounts and grant privileges as needed to select, analyze, print, edit. publish, etc. the data when required.
4. When a user changes roles or leaves the organization, simply alter or revoke privileges as needed, or add/remove their participation in a database role.
5. This model can be further controlled with the use of versioning in a versioned environment.
If your business needs require most users to regularly create and own new tables and feature classes instead of working on a planned geodatabase schema, then I would recommend putting in place a policy in which user accounts are disabled at the database level for a period of time when his or her role changes in order for a data migration to take place to another schema. This policy leads to collaboration with your DBA or IT Management team, and Esri Technical Support is available to assist with any problems that may arise during data migration.
I hope that is helpful. Please let me know if I understood your question properly.
-Matt