The servers at the launch of 2.0, unless you were on a Legacy server, were dead. Yeah, they had small populations, but only because if you weren't on a Legacy server already after the 1.0 to 2.0 shuffle, you had to choose to create a character elsewhere. They took off that lock and people were free to make on whatever servers they chose, and people flocked to... the Legacy servers.

When the servers get super high-pop again during a patch or even 7.0's release, I guarantee people will go "f[kupo]k that noise" and transfer off. During Stormblood's release, people transferred off Balmung and Gilgamesh because a literal unmoving queue of 10k people stopped people from being able to log in after 1pm PST until 9pm/10pm PST, among other things (looking at you, Raubahn Savage.) Around that time, they started offering incentives to leave, and people took it. They moved to the quiet servers and now all of the NA worlds (this was before the split of some worlds into Crystal) have a healthy population. It takes a lot of time and literal frustration for people to move, and when the inevitable "the queue is as long as a work day to get into this server" happens, people will move.

The new NA DC will start filling out, just like the EU one did. With the OCE DC, I think that'll take a bit more: namely, people advertising that one to be on if they're in OCE and getting a healthy population from new players and old players alike.