It's also a bit bothersome if you're there. The people who stay are probably aware of the alternatives, but also don't feel like moving, so people going over and trying to get them to join Kbin/Lemmy are the equivalent of people asking you to join the Church on the street. Having the alternate community on the sidebar, or if the sub shuts down is fine, but probably not a good idea to shove things into people's faces.

You're already aware that the Church exists, and the advertising is not going to make them want to join it any time soon. If anything quite the opposite, or it'll make them want to cause trouble just out of spite.