Regarding your point about spam and bad actors: even though it's not the solution we've ended up with (i.e. training spam models, creating rulesets, sharing blacklists etc), this problem space always makes me think of Microsoft's 'Penny Black':

https://en.wikipedia.org/wiki/Penny_Black_(research_project)

There are further economic problem there; people won't want to pay anything for email, for example, now that they're accustomed to having it for free.

But if sending a message came at the cost of a few smartphone GPU cycles -- or some other widely accessible but difficult-to-hoard resource -- and as long as it didn't affect a person's mental perception of their bank balance -- perhaps there'd be opportunities for something like Penny Black again.