I went on Forge DB to do a bit of research and it seems that they are really not that far off base by opening another world.
It seems their target for active players on a world is about 7K. So far it seems they have done a good job at filtering new players into worlds to keep them more unified than I would have thought.
Here are the numbers of active players on the US server by world, the EN and also beta.
A- 8K
B- 7K
C- 7K
D- 7K
E- 7K
F- 7K
G- 7K
H- 8K
J- 7K
L- 7K
M- 8K
N- 7K
O- 7K
P- 7K
Q- 7K
R- 7K
S- 7K
T- 7K
U- 7K
V- 7K
W- 6K
X- 7K
Y- 4K
Z- 3K
UK
A-7K
B- 7K
C- 7K
D- 6K
E- 6K
F- 6K
G- 6K
H- 7K
J- 6K
K- 6K
L- 6K
M- 6K
N- 6K
O- 6K
Beta
4K
So while En does have less worlds, the active player counts shows that there is yet not need to open another. Though there is only one beta server, active players on that world are much less.
While Y and Z numbers are much lower perhaps forecasting new sign ups deems another new world necessary to keep with their 7K trend.
All in all the actual numbers were nothing close to what I expected and while I still don't like the double AA confusing us all, it does seem they are not pulling wool over eyes to say they need another new world, at least not completely.
Perhaps they decided to do it early for quarterly earnings but in any event the numbers aren't that far off.