You arw too optimistic, there is always a way, and it will probably not replace lemmy or kbin but maybe mastodon or any other healthy ActivityHub service. The danger is there, just because you don’t know the answer to the the real danger doesn’t mean there is no danger.
Facebook (or rather “Meta”) has a buisness model, and its there to damage us all, grow into infinity and be the number one. They will become creative and always take an unhealthy turn whenever its possible.
With very little probability it may become an utopia where everything will be nicely federated and still Open and Open Source. With high probability they will focus on full destruction to gain the most money out of any situation.