I agree with most of your reasoning except this conclusion: “As currently conceived, decentralized social media relies on one main premise: that users will pay each other for content”.
Also, it is worth noting that any peer-to-peer system is decentralised, regardless of whether or not it has associated crypto-currency.
I would argue that it makes sense in any community, for all users to be rewarded for all actions adding value to that community, and to be penalised for all actions reducing the value of the community. If the community is non-profit, then there is no need to charge anyone for anything. Everyone can derive a continuous income by continuously adding value. If the community has a crypto-currency, then the system of the currency can be automatically geared to reward / penalise as appropriate.