The challenge is getting the market to value the token that miners are paid in, such that the chain is sufficiently protected by their proof-of-work. Digital money may be the only thing valuable enough to absorb the capital loss associated with the energy spent on proof-of-work, whereas a messaging board -- or any other application that creates relatively little value -- will not generate enough profits to cover the miners' work.

The only reason the Bitcoin blockchain is mined so heavily is because of its tokens' high price -- and sufficient market depth, which can absorb miners cashing out their earnings -- which basically sets in motion something that can't be stopped until it reaches non-profitability, unless the entire world decides to forego profitable Bitcoin mining.