Having dipped my toe in some of this stuff I think it absolutely sounds like a ten-billion dollar proposition to offer a turnkey solution to the problem.
That's why there are dozens of startups in the auth space.
Turnkey is harder than you think, though, because authentication, while undifferentiated in general, does get tied up in business logic. A multi-tenant B2C SaaS has different needs than B2B on-prem deployed software, to name just two use cases.
