I think it's more accurate to call them "digital goods", because IMO what enables the less than near-zero marginal cost (I consider them negligible costs) is the digital nature of the "merchandise"
(a jet engine is a technological good, but it is material, not digital)
other than that, I completely agree. The trend towards centralization comes from the economics of the market (not from the nature of technology)
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.
Hashcash has the features you described above, but no mechanism for "returning" PoW to a user.
Exactly. It's not a technological phenomenon, it's an economic one. It's actually present in all markets, but traditionally there is a "local" effect that allows variation to exist (if it's not free to ship goods to anywhere in the world), or a negative return to scale.
Technological goods, especially SaaS, can be "sold" anywhere in the world for near-zero marginal cost and tend to have positive returns to scale. Conventional economics dictates that this will tend to a monopoly. https://www.quora.com/Why-is-increasing-return-to-scale-inco...
Decentralists also tend to to recognise that, for most users of communications services, having a central authority to discipline bad actors results in an improvement to the service. This didn't exist for email so the Spamhaus and related services had to be invented.