Yup, weeds. The on-chain/off-chain distinction is the main thing. Whether it is a mempool-like thing or a separate database or something else is definitely weeds.
As far as the system being responsible for paying, I could imagine some sort of setup where if these pseudo-posts from pseudo-accounts get votes and earn, then the rewards could accumulate in a pool or holding account or something which could then pay for accounts/bandwidth.
But the problem is still that many users are not going to earn significant rewards, ever. I'm just not sure the 'earn your way to pay your way' model of providing bandwidth is viable at all. Some users are going to be content producers and some (probably most, by a lot) are going to be content consumers. The latter need their bandwidth paid for from some external source, somehow.