Yes, but this opens up another problem with a federation controlled by the server admins and not the communities. Trust can be withdrawn as a punishment or due to a disagreement or just different views.

We just saw that with instances defederating others due to incompatible views on politics. I expect more of that for much smaller disagreements until its just clusters of like-minded people in their own bubble. At least I want to see what others say that does not agree with my own views and values - how would I keep a realistic perception of reality otherwise? If I stay in my bubble too long then I might start thinking "everyone" thinks foo=good and bar=bad, while it might be the opposite.

Other networks like freenet use a wot, but for each user. TOR does not filter out relays, but allows its users to do so. And, yes, they all have their own issues with their approach.

What I am trying to say is: I had hoped for the fediverse instance admins to not consider themselves as lords of each their own feudalistic dukedom with "trade agreements", but instead to consider themselves as mere service providers for the greater good, sworn to neutrality when it comes to opinions being discussed (abiding to law where required to not get sued or worse of course). Our strength lies in the federation network itself, without it we would just be a bunch of forums. If we allow the network to fragment more and stop talking to each other, the monolithic pseudonetworks of the big corporations will stay in power.

I know this might be unachievable, or even undesired, but at least a web of trust that is controlled by its users, instead of the admins, is much more appealing to me.

Hashcash would slow spammers down without troubling regular users too much. It would be scalable and with a meld-based algrithm it might be future proof. It could even complement a wot.