Well implemented network hardware can have high bandwidth and low latency. But that doesn't get around the complexity and headaches it brings. Even with the best fiber optics, wires can be cut or tripped over. Controllers can fail. Drivers can be buggy. Networks can be misconfigured. And so on. Any request - even sent over a local network - can and will fail on you eventually. And you can't really make a microservice system keep working properly when links start failing.

Local function calls are infinitely more reliable. The main operational downside with a binary monolith is that a bug in one part of the program will crash the whole thing. Honestly, I still think Erlang got it right here with supervisor trees. Use "microservices". But let them all live on the same computer, in the same process. And add tooling to the runtime environment to allow individual "services" to fail or get replaced without taking down the rest of the system.