When you go from a traditional IT operation model to modern based on Azure AD and Endpoint Manager, you will have a migration period with resources in both camps. Typically, endpoints first go to the cloud while well-established services lag behind.
This does not have to tie the endpoints to the ground. You can move your computers to pure Azure AD join, and still have access to on-premises services in Active Directory as long as the identities are hybrid.
Some tend to use Hybrid Azure AD Join (HAADJ) since they have some legacy traditions of device and application management. HAADJ can thus be tempting, but in the long run it will give more headache than pleasure. The best approach is to move the endpoints to pure Azure AD Join devices and then put more effort into adapting to the new operational environment offered by Microsoft Endpoint Manager.