I would keep Dwilight. It's quite a bit different that the other islands. The odd geography and long travel times have given rise to some truly unique cultures like Luria and Sanguis Astroism.
I also feel that it is one of the better islands for retention, based on personal experience. Many of the older islands feature realms with static political cultures and entrenched characters with virtual monopolies on power and influence. Some of these realms (and even entire islands) have effectively become the personal sandboxes of a small handful of players. When I first joined the game in 2008 with a small group of friends, I joined a realm and island such as I am describing. The culture I first encountered there was a huge turnoff, to put it lightly. Within two months I had left the island with my friends and emigrated to the then-new Dwilight. Had that experience not proven to be significantly better, I likely would have quickly given up on the game.
The long and continuous history of BM islands, while in some ways a boon, is detrimental to the health of the game in other ways; on many stable islands meaningful turnover at the highest levels is virtually non-existent in some realms (if not most realms), and all the plum positions simply rotate between a set cast of characters that changes only rarely. One of the best things about Dwilight (and also Beluaterra before the Invasions were officially ended) is that this dynamic has yet to set in (or in BT's case was completely upset every time an Invasion happened). In both cases, opportunities were and are there for newer players to join realms that truly need them, not as cogs in a machine, but as leaders and story-makers in their own right. That's the opportunity that I had with Astrum, and it is definitely what hooked me to the game. I eventually had similar experiences on Beluaterra. I have notably failed to find such opportunities anywhere else, despite (in some cases years-long) sojourns on the stable islands, excluding the Colonies. I even had a ruler on one of the stable islands once, but it wasn't much fun, it was mostly a chore. Thanks to preexisting power structures and entrenched relationships on the island, I had virtually no ability to make a difference. There was nothing I could do as a ruler that mattered at all in the long run, so I found it to be totally uninteresting and gave it up within a few months.
My point is, Dwilight (and maybe BT still, though BT has started to stagnate now) bring something to the game that is different than the other islands: A more welcoming, dynamic and open-ended experience. Change is still possible (and frequent), turnover in the leadership positions in still relatively high, and alliances and relationships between realms are still shifting to an extent not seen in most other places.
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