You should always direct people to a landing page where they can choose from or be directed to multiple different instances.
https://kbin.pub/en or https://kbin.fediverse.observer/list
https://join-lemmy.org/ or https://lemmy.fediverse.observer/list
You can direct people away from specific instances if, for example, they have been de-federated.
You are right that there are downsides. But my main concern is load balancing. Part of the reason lemmy.world has been cut off from Beehaw is BECAUSE too many people joined that one instance and now it is out of control.
The question is, are they generating enough content without that external source that it wasn’t felt much? If that’s the case, then it shouldn’t matter. If it isn’t, then people will leave and go to where the content is. Frankly speaking, my thought is that the federation should only be on as a backfill or last resort- if you’re unable to generate content and discussion organically, import someone else’s discussion.
Okay, but the same thing would have happened if they all joined Beehaw directly, or were spread out among many many instances. Part of Beehaw’s logic was “Well, a disproportionate amount of moderation had to be done to lemmy.world so we defederated”, but isn’t that simply because there are more users on that particular instance? If, for example, we assume every single lemmy.world user joined separate instances and behaved the exact same way, would Beehaw have defederated all of their instances? If not, then the response wasn’t the correct one, in my opinion. What happened is that Beehaw saw their mod work increasing, and nuked a large portion of their active userbase (federated users from non-Beehaw instances) to cut it down to try to maintain the culture they’re trying to build. Unfortunately, I think all it will have done is throttled their growth and Beehaw will become an isolated instance with no large communities on its own, because the communities that were there (that lemmy.world was participating in) will just be recreated on different instances.