Is there a tipping point where it’s a net loss? If I understand the protocols correctly, the whole back end federation part of the equation is push based, so if everyone was running their own instance, lemmy.ml would have to push every post to every individual instance in the network. At some point isn’t it more efficient to only have to serve posts when people come here to look at them?

Lemmy is just the latest in a very long line of potential reddit successors. Historically, you can’t move a subreddit to a different platform because redditors are users of reddit, not users of your particular subreddit.