Azure AD Connect is the replacement. In the architecture you described, MIM is going from <other source> into on-prem AD, and AAD Connect is carrying data from on-prem AD into Azure AD, right? In that case, MIM's only functionality that is required for you is to go from <other source> into on-prem AD.
My understanding is that for quite a few years now it has not been advised to use the WAAD(Windows Azure Active Directory) connector in MIM for any new deployments, but rather to use Azure AD Connect. Given that understanding, you should only be using AAD Connect moving forward, and then you'll have to solve the problem between any external data sources and on-prem AD separately.