If they pull the plug on ActivityPub and take their users back, things would just be exactly how they are now. Since they can't create communities or magazines like we can (and it's very unlikely Meta is going to try to implement this), if they want to participate in discussion here, they'll be posting in our communities. Kbin's magazines are uniquely suited to this as well because content gets sorted into them based on hashtags, so they wouldn't even need to know that they're posting to a magazine to do it.
We're already in a situation like you describe though with lemmy.world's near monopoly on large communities, which seems concerning to me as is.
testing