Is there any reason this has to be true? Probably some majority or significant minority of mobile devices spend some eight hours a day attached to a charger in a place where they have the WiFi password, while the user is asleep. And you don't need 100% of devices to be hosts or routers, 10% at any given time would be more than sufficient.
Except it don't. Route and content takes hours to converge.
If a peer says "hey there's a new version of this" and that peer also has pinned that version, then I can get it from them right now, well before the network converges. Yeah maybe it'll take a few hours for the other side of the planet to get the word, but for most data a couple hours or a couple days is fine. Tolerating latencies was kind of the point of calling it "interplanetary".
What's the use case where I'm on the other side of the planet and I somehow end up with a CID which I can't resolve? How did I get that CID so much faster than content to which it refers?