> what about when we’re making multi-year journeys to Mars and we need a relay network of “space data centers” talking to each other, caching content, etc?

How would this work? Planets orbit at different speeds, so you can't build a chain of relays to another planet. I can imagine these things orbiting planets, but is that worth it compared to ground-based systems?

We'd build it then? The problems of a space data center are extremely generic and only worth solving when you actually need one. Which would never be in low earth orbit.