[deizel]

> ... anyone who wants to enlarge its available message space ...

Bear in mind that the proof-of-work algorithm actually aims to reduce chatter on the network. Every ten minutes, a single Bitcoin full node broadcasts: "Hey, this is the latest state of the ledger, and you can trust it because [insert proof-of-work that was calculated out-of-band]." So, as the network's processing power scales up, the computation difficulty is also scaled up, so that the "message space" required for consensus remains constant.

> That seems rather limited for a growing population's needs for communications around an increasingly valuable resource base.

While the way distributed consensus is achieved (see "Byzantine Generals' Problem") consumes most of the network's processing power, it's what makes the resource valuable. However, since this is done in parallel, it hardly consumes any of the network's bandwidth, freeing up that "message space" for important things like sending/receiving/propagating transactions, which are much less demanding computationally. All that is required there is some elliptic curve cryptography (ie. a valid public/private key pair + any low-powered device with correct software and an internet connection.)