lemmy users want to stay on lemmy. they don't want to come to kbin (even to troll). and I think the few that might end up here will just be drowned out by good faith kbinauts.
Given the much higher number of Lemmy instances, you may well be 100% correct on that, that is, even trolls may just decide to 'stick with what they know' and continue to use a Lemmy based instance.
if someone has an issue with kbin being open to everyone, then they will naturally block us no matter what we do.
Also correct, however, I understand that if a post is local to kbin.social, and had comments from lemmy.world users, in addition to kbin.social users, and a beehaw.org use came to the post, they wouldn't see any of the comments from the defederated users. That is to say, so long as one instance doesn't federate with another, they will never see posts from that instance, regardless of where they are.
I'm not saying that won't still prevent people defederating with kbin.social purely because we're open / federate with another community they specifically defederated from, but there's less reason to if kbin.social maintains a quality userbase.
I think that spirit and culture is far more important than whether or not a few bad apples start getting other instances wanting to defederate.
If a large amount of "beehaw haters" start flooding kbin and changing the culture to be one of toxicity and vitriol, then I do think we'd start having a problem.
Another agreement from me here, however, the point of this post is to start a discussion around whether or not we can prevent being overwhelmed with bad apples too early on in the platforms lifespan (although I appreciate we've already addressed this above).
As has been said, at this point, it's entirely up to @ernest, but I think it's worth looking at potentially taking a couple more people on to assist with the moderation side of things.