Penchant and Ketchum mentioned that many play on Colonies because they don't have time to play on the other islands. So perhaps closing AT would result in a bigger influx to EC and FEI.
So AT=>EC+FEI, BT=>Dwilight. A very high influx of density to the core islands. If we are going to close islands, might as well do it fully and gain the most benefit possible. One big pain instead of still having to deal with this problem a few years down the road.
I feel that SMA and 1 char per island are issues tangential to the one at hand. I don't see a reason to change them, but those discussions don't influence which island to close as much.
P.S. For the record, Dwilight=>BT and EC=>FEI+AT is an equal proposition in my opinion as these are examples to think about what would happen and what the result might be like.
[EDIT: These scenarios are worth considering as possible results, the individual islands, which is closed and merged into which other one, matter less in this discussion.
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