Non-CDB database vs Single tenant CDB
Having a single tenant CDB allows you to plug/unplug on upgrades, you can use the clone function and it allows stricter separation of admin users in the CDB and the PDB users.

You find this arguments in multiple articles and presentations of Oracle, all hint to the fact that you also should get used to the new normal. 

https://blogs.oracle.com/multitenant/single-tenant-configuration