There are plenty of languages and RPC frameworks where you can solve this without resorting to a service mesh.

Practically, and to your point, service meshes solve an organizational problem, not a technical one.

On that scale I'd expect people to use client-selected replicated services (like SMTP), and never something that centralizes connections (no matter where it's close to).

You can always add observability at the endpoints. Unless your infrastructure is very unusual (like some random part of it costing millions of times more for no good reason, as on the cloud), this is not a big challenge; you add it to the frameworks your people use. (I mean, people don't go and design an entire service with whatever random components they pick, or do they?)