In essence, what you are saying is 0x60 will become something similar to a paid forum where members will have to subscribe in order to interact or reply to posts. I don't know how will you promote this site to potential users when the whole internet is used to freemium models.
May I suggest that you consider the BETA with Ropsten ETH as the live one? Meaning if there are at least 1,000 - 10,000 users in beta, you port the content into the live one. Kind of like "post now while it's free."
To view comments and topics you just have to establish a web3 connection. It doesn't cost anything.
The cost per comment depends on current network conditions so it can fluctuate. And it's not that high in my opinion for it to be a major problem if a user feels like they have something important to contribute, but at the same time it is high enough to stop mindless spamming.
BOARD token is also a deflationary token which hopefully will help the project gain traction because there are a ton of deflationary coins out there with no actual use case, yet people still want them. In the end it all depends on whether users feel like the price per token (Gas) justifies itself in the long run. Nobody can guarantee there will be any value in it of course, but hopefully the deflationary aspect will help.
I also thought about your idea of leaving it on testnet at first. I guess the best we can do is offer testnet tokens to be converted into main net tokens 1:1 once we are ready to go live?
Well, I guess that's better than nothing. Anyway, you know what I'm talking about. Even people here in Bitcointalk don't know what a web3 browser is. (browser that can connect to DApps, currently through Metamask). I'm seeing the forum is going to be a niche (DApps interaction) within a niche (crypto-centric internet users). 0x60.app is truly five steps ahead of its time.