In highly distributed microservice architecture, there's almost never a single upstream. In some cases you may have a couple of customer-facing entry-points (a global API gateway, and a couple of BFFs), but these are not the only paths that need to be protected.

There are client-side retries (which have broken GitHub in this case) and server-side initiated API calls between microservices that don't pass through any of your ingresses (e.g. triggered by an ETL pipeline, or a scheduled job).

With a complex architecture you can't just slap a circuit breaker on a couple of ingresses and call it a day. Don't get me wrong, putting them there does go a long way, but you won't be covering all your bases.

Doesn't do you any good if the outage is in the circuit breaking layer, which it was for GitHub (this started as a load balancer outage).