What you are talking about is in a large part authentication. You can do authentication using an external service and still have your user table locally. You can also do authorization locally with a local session table while leaving authentication to a SaaS.

All I am seeing here is Django modules

Social logins, multi-tenant and organizations are very far from table-stakes for an MVP.

Whether it's painful to put in later or not is sadly nothing that the managers and executives concern themselves with.

By the time you're so big you need all of that, there will be other people at the table to "hack that in".

Django, Rails etc handles this.