So will we lose all our posts and other things in the event of this happening?
That would depend on how it's implemented. If the goal of the pitchfork is to drop a single field, or change the characteristics of a field you'd have to use a pitchfork, but then everything else comes over.
On the other hand, maybe you wanted to remove all of the posts from the blockchain and use some other storage strategy.
It depends entirely on the problem you're trying to solve.
In the case of an app like Steem Monsters, it would be pretty "easy" (though non-trivial) for them to just transition to the pitchforked chain by deriving the data from original chain, and not rely on the custom_json migration to make the transition for them.
Dropping content by certain types of accounts could be a smart move to reduce the overall size of the chain. 40k account farms do add up.
If something like this were to happen, I hope we'd get a few weeks ahead of hearing about this decision. I'd like to save my posts done in here, if this isn't the valid place to store them anymore. :S
the posts are not going anywhere. They are stored on the blockchain. There a decentralized copies of that. You also could download markdown versions of every of your posts with this https://steemit.com/steemdev/@holger80/store-all-posts-from-an-author-in-markdown-files script