I’m going to comment on this, but I don’t really feel qualified, so my comment will be more broad than the particular context of Swarm, but I’ve always felt that people have a massive hole in their thinking related to incentives.
I call that massive hole “usefulness.” The historical Ethereum state is useful. If it were easy to access by individual community members, and they could easily get only that portion of the state that they themselves need for their own unknowable purposes, that would be incentive enough for them to store it (and, if the system worked as it should, store just a bit more than they themselves need, so they can share the state with others).
Incentives don’t necessarily have to be monitory or tokenized. Look at books in a public library. Why do public libraries exist? What’s the incentive model / business plan?
Usefulness.
Historical blockchain state is more like that than some sort of digital product that needs to be “provided by someone.” It should be used-by and provided-by us all through a system that is designed that way on purpose.
(Shameless shill: this is exactly what the Unchained Index works.)