The problem with using extant votes is that votes are associated with a given post, not associated with a given provider. Who should the upvote go to? The curator who voted it up? The original author? All of the curators who voted it up? All of the above? While any given answer is relatively arbitrary, it really should be the decision of the user. (That also means that "why am I seeing this thing?" needs to be a more transparent and answerable question.)

If you continuously rescale Trust values to be between 0 and 1, normalizing to a float based on a moving window, you can integrate some sort of decay function over time. This also solves the problem of your interest in a given provider changing over time. More recent Trust investments will be larger than older ones.

Every system is exploitable. Every single one. Web of trust systems are vulnerable to actual interpersonal negotiation exploitation. That's a problem anytime you have humans making decisions. Nature of the beast.

One assumes that in a system with a nontrivial number of users who are creating content, they should see some random new content from users with no connection to their personal web of trust once in a while. Of course, you can make the option to do that pilot deliberate choice, going to a specific page or tab, part of the site design. I suspect it wouldn't be terribly popular, however. (See the current New tab as it is.)

10-year-old technology. It's amazing.