1.) Pay for SnapPro 2.) Sit through advertisements 3.) Let people who want to pay for SnapPro pay, while you don't because you don't want to pay for it. Therefore no ads, and Snapchat is making money.
Option 3 is the best for consumers. Option 3 is possible if the price-point dictates that enough people will pay for it to remain alive and well. Not everyone will pay. But when is that not always the case?
Isn't this the sort of thing that could be implemented as a peer-to-peer networking service?
You can only talk about the "all but 0" marginal cost when you can produce a piece of software that really does have an all-but-0 marginal cost. And, therefore, doesn't require the computer running it to be always on, or run on a dedicated VM, etc. Sure, such a thing may be possible, but you have to produce it first.
Unfortunately.
There are plenty of distributed designs which could be secure and "read once within a time period".
(btw, a distributed network topology can look isomorphic to a centralized topology at the application layer. You seem to have a highly simplistic map of the territory.)