Yes, but for offload, all that matters is your local lan swarm.

> A room full of people using their phones is what I have in mind.

That might work to some extent, but how often do you (i) spend your time in rooms full of people and (ii) have no wifi.

> Anyway, the point was whatever’s the offload factor it would still be a net positive.

Not quite. If the offload factor is sufficiently small there is no rational reason to burn battery on it.

> Also one can imagine if IPFS gets popular, just as ISPs collocate CDN caches in the present model, WiFi routers could come with a local node of their own. The “Web Accelerator.” And now we’re talking!

Possible, but unlikely. Web cache hit ratios are abysmal, normally it's much cheaper to just up the bandwidth. A lot of business models would also have to change for the (large) and interesting content to become third party cacheable.

> There’s opportunity for caching at many network levels, but there’s no incentive to do so with HTTP.

There are also many business models where there is no incentive to cache either.