If we have to implement all those things, is there really much of a remaining lift in implementing OIDC, apart from bringing in a library into a web app?
OIDC is great for greenfield and much better than writing user signin flow every time.
That being said, if you're working on a greenfield app where you need auth, using an OAuth/OIDC server is much better and faster than creating yet another membership system for that app. That's a major advantage of identity federation.