I think a platform like this is as impractical as you are thinking. However, there are potential other things you can do by mixing centralized services with decentralized ones. You could create a forum like hackernews that utilizes traditional infra but restricts access to users that hold NFTs or something similar. Think of something like a Patreon type deal where access to the service is tokenized. I've been kinda thinking of stuff I could do with a NFT project I've been working on and it's a thought I've been bouncing around.

There's no way I'd want to create something that is similar to what you're thinking, because it's pointless as you are thinking. Nobody would pay for this. Also messages such as these have no reason to be on an immutable blockchain. But you can certainly create specialized services for people that support your projects using ideas like this quite seemlessly.