All those moderator actions you described would still be possible in web3. Moderators would still be able to up a topic, merge a topic, delete topics, ...
But:
* The fact that the moderators have access to these mechanisms would be known by all users
* The use of these mechanisms by moderators would be public. If a topic is upped by a moderator, it would be logged in the blockchain. No moderator would be able to do it secretly.
Regarding the fee, it's true that it's a big downside. To get a feasible use case, the devs would need to find a blockchain that reduces them.