I think the fee can be reputation based. It start out very tiny, something like 100 satoshi per post, anyone can afford to make a lot of posts. BUT the quality of your posts (determined by upvote/downvotes) will affect your "reputation", if you have a bad reputation, then the system raises your fee to much higher. Also if you have a good reputation, your fee is reduced, and also you occasionally get paid by the system for making such good posts (using the fees received by the system).
It is still avoiding the basic issue, why would anyone pay anything when posting right now is free?
If there is some specific application then it might be useful to a small group, but then they will only trust someone and can set up a centralized server. Decentralized forum sounds good in principle but doesnt make much practical sense.
Because as I said in my post, "if you have a good reputation, your fee is reduced, and also you occasionally get paid by the system for making such good posts (using the fees received by the system). "
So even though there is a tiny fee, you could potentially make money if you post high quality posts. I have seen a lot of people will spend their time online doing these type of things, like promotional twitting, or making blog posts, for pennies in pay. This will be something similar.
High quality posts attract more readers/participants, vast majority of people are consumers of high quality posts, so they might be willing to pay a fee to read some posts, it's pretty common for forum posts to have a "forum currency" attached to it, and to view the post, you have to buy it (I have bought and sold these forum currencies previously, for real money). Also many forums, like private trackers, will charge a fee to even register for their forum. This decentralized forum will be perfect for such purpose, it'll avoid any legal enforcement concern over hosting an illegal private tracker forum etc...
