> focus their efforts on ensuring downstream IO doesn't get overwhelmed (db read replicas, caching, etc) before optimizing runtime performance or autoscaling out unnecessarily.

All good advice, but the choice of runtime can affect the point at which autoscaling and load balancing even need to enter the conversation at all. Optimizing, say, a mostly in-memory cache service and writing it in Golang may yield results like "we can run a single instance of this and serve three orders of magnitude of business growth; slap it behind a DNSRR or a k8s NodePort for update/replacement/fast failover if it crashes, no complex load balancer needed", where writing the same thing in, say, PHP might require discussing orchestration/load balancing/memory/worker process recycling/autoscaling early on in the service's lifetime. Being able to skip those conversations (entirely or for a long time) is a very significant business benefit.