Some information has value that remains relatively constant over time: historical records, literature. Some information has value that decays slowly: basic materials in the sciences, large bodies of well-established measurements.

Some information has value that decays fairly quickly: current scientific progress that invalidates older measurements or has better predictions than older theories, current events, prices of common items.

And some information has value that decays extremely quickly: weather, financial markets, prices for things that you want to buy or sell now, casual interactions with other people.

If you build a system that can connect fast enough to the Internet in terms of latency and throughput, you don't need much local caching. That's what we have in current smartphones.

Without a subEtha network, having a large local cache becomes increasingly important the farther away you are. But without a mission profile to plan for, estimating things in terms of current storage technology is as useless as coming up with a security plan without a threat model.