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.