[echelon]

100% this.

And to your point about client libraries: for any shared behavior that needs to be local to the app (eg. session handling, permissions, etc.), it's more appropriate to spin up a sidecar that you can talk to over sockets than to try to build client libraries in each and every language. Your client libraries will differ in behavior and it is an incredible pain keeping them all up to date, patching every app, etc.

Envoy for S2S and traffic mesh + sidecars for shared behavior is better than building client smarts.