I agree with the general principle. Fewer moving pieces make for more stable applications ("choose boring technology"[0]).
However, I was wondering what you do when you have more than one application that the same userbase wants to access. I can see 3 options:
1. make them register/have credentials for each application (not a great user experience)
2. use a standalone auth server and deal with the increased complexity
3. pick one of your applications to 'own auth' and have the other applications delegate to it. congrats, you've just invented a standalone auth server that is coupled to one of your apps
What am I missing?