Unfortunately, there are only two ways to achieve such a feel on an ongoing basis: continually opening new continents, and performing periodic resets (in whatever form) of one or more continents.
Earlier in Dwilight's development it may have been possible to extend the colonization phase. Tom manually removed existing monster groups from the map on two separate occasions if memory serves after some people complained about it being too hard to start a realm. If he hadn't done that, things might have taken longer, possibly a lot longer. I remember the before and after difference being dramatic in Astrum the second time he did it. We went from being under constant pressure to being left almost completely alone for the better part of a few months. That's what allowed us to found the first Niselur and expand it so quickly. That Niselur may have fallen apart, but it was never the same afterward. The monsters never really recovered the numbers they'd had before, and we grew stronger during the respite.
If a conscious effort had been made to beat back the waves of colonization periodically that could also have extended things, but doing that would have been similar to what was done on BT, as you say; you'd have to create some sort of disaster periodically that partilly reset things. Still, it can't be too difficult to just spawn several large hordes of AI-controlled monsters every now and then and let them do what monsters do. It wouldn't require the kind of complicated storytelling and GM-controlled NPCs we saw on BT from the Third Invasion on.
