I would disagree here. You probably need OAuth with popular social services and implement username, password or OTP-based auth overall. For an MVP, you don't need to care about more details beyond this; it is hardly 10% of the entire effort, if not 5%.
Social logins, email logins, password resets, multi-tenant, organizations, many to many users to organizations, etc etc. Not necessary for MVP, but can definitely be painful hacking in later if the MVP hits.
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.