Maybe we should draw inspiration from the real world. How did most of the modern Europeans come to Europe?
Barbarian invasions.
Perhaps RoB could be replaced with multiple
mobile tribal totems. When forests start recovering on their own (and it makes no sense whatsoever that they don't), newbs could always be kept on land with resources by moving their spawning point. Of course, the whole moving business is a bit tricky to do right, as it could be used for griefing. It should either require multiple lawspeakers (which is still exploitable, but require a lot of effort), or the totems should in fact slowly move on their own when the land around them is depleted (say, unclaimed tree count is under a certain number). The latter would represent barbarian invasions quite well, I think. Of course, they should never appear directly on village territory or tribal claims, but even if a horde of newbs appear in your neighbourhood it can be bad news because the forest around you is as good as gone. This would only be a temporary setback, though, because forests would slooowly regrow on their own.
In addition to these "wild" totems, maybe multiple villages should be able to form into a tribe and build a permanent RoB-like spawn point. Newbs might be a huge pressure on natural resources, but they're also a valuable resource themselves if they can be turned into productive citizens. This RoB-equivalent would let villages allow newbs close to them for evaluation purposes while not actually letting them into the village where they can cause problems. It should also be possible at this RoB to request membership in any of the villages that make up the tribe that owns it. Something like runestones with village names and short descriptions, and when you right-click on it you get a "request membership" option, which the lawspeaker of the village in question can grant or deny at any time from either his village idol or this RoB. There could also be some management options, like setting immigration quotas, banning certain bloodlines from spawning there (so a griefer would have to start from scratch every time) etc.