Regarding your point that scammers will just spin up more accounts:

In a world with potentially unlimited swarms of scambots, perhaps we’ll need a system where we simply ignore all unvetted users. But of course we don’t want the isolated user who’s a real person to be left out in the cold. So build multiple methods to break into the system:

- pay some fee; you get to decide how much is enough to get onto your feed

OR

- social vetting: one or more trusted users attests: “this account is a real person, I know bc we communicated in meat space.” This feeds into a score, and you decide what threshold score is enough to break into your feed.