That has nothing to do with the fact that if your systems are distributed, you will have eventual consistency.
If System A needs to tell System B about an event in order for A and B to remain consistent, but B is down, you've got eventual consistency, because B can't become consistent with A until it's back up and has performed whatever recovery is necessary to process that event. Service discovery does nothing to solve that problem.
In addition, the network isn't just up or down. It's varying shades (dare I say, 50 shades?) of down or broken. A single machine might not be accessible due to a switch issue. An entire rack or aisle might be compromised by a bad router or faulty routing table. A network cable might be flaky. The truth is you just don't know, and that's all inside a single LAN.
Your service discovery system could be able to see service {A,B,C}, but service A can't talk to B or C due to network issues. It happens.
