I was thinking the same thing.

Dynamis would've just been what Shadow for EU was. A temp DC to deal with the initial surge of players during expansion launches. As others brought up, they probably seen the Shadowbringers surge thanks to the WoW (temporary) exodus and thought the players would stay. Fairly certain between the turnaround WoW has done (most of my WoW friends who migrated have returned with latest expansion) and unless there's some major changes in development (even a casually skilled player as myself is struggling to find reasons to log in short of just doing society quests to level up jobs. Don't even bother with Roulettes since thanks to the even longer patches have 2 jobs at full tomestone gear and leery on doing Jeuno until 7.2 and new tomestone gear to make it less lethal for mistakes) I really don't see numbers climbing to justify needing four DC's for NA. As suggested, splitting the servers up and merging them into the current 3 could salvage the situation.

Only problem I see is if they announce in advance which servers go to Aether you'll see a flood of transfers to them to get to be part of the big raiding DC that's been and probably will be on lockdown until who knows when.