But there's really nothing "cryptographic" about an isolated authentication service. To drive the point home, and
don't do this, but if you (1) used dedicated hardware to run it, (2) IP filtered the box down to just HTTPS, and (3) ran the service using Go, Rust, or Java Servlets, you
probably wouldn't even need to use a good password hash.
I'm only talking about the AuthN problem, by the way. I'm not making a general argument against circuit breaker architectures.